feat(tracker): ApplyTrackerFacts reducer + shared observer skeleton (#112)

[harshitsinghbhandari]

Closes #112.

Lands the two scaffolding pieces from the issue so the Tracker observer (issue #35) and its provider adapters can be built in small, reviewable follow-ups instead of one monolithic PR.

Part 1 — ports.TrackerObservation DTO + lifecycle.Manager.ApplyTrackerFacts

• New backend/internal/ports/tracker_observations.go mirrors scm_observations.go's field-level documentation style: Fetched bool, ObservedAt time.Time, Provider/Host/Repo, normalized TrackerIssueObservation, Comments, and the Changed{State, Assignee, Comments} discriminator.
• lifecycle.Manager.ApplyTrackerFacts in backend/internal/lifecycle/reactions.go mirrors ApplySCMObservation shape: Fetched gate → terminal-state shortcut → per-bucket reactions. Three reactions:
  • Issue state done or cancelled → MarkTerminated (idempotent).
  • Changed.Assignee → log only via slog.Default(). Per-policy reaction is reserved for Tracker: mirror agent lifecycle to issue via Comment + Transition #40.
  • Changed.Comments carrying bot comments → one-time nudge with bodies concatenated; deduped by comment IDs via sendOnce with empty prURL (in-memory only — issue-row persistence ships with Tracker observer loop: poll + ApplyTrackerFacts into LCM #35).
• Unit tests in backend/internal/lifecycle/manager_test.go cover terminate (done + cancelled), log-only assignee, nudge fires, nudge suppressed on repeat, new bot id refires, not-fetched no-op, terminated-session no-op.

No observer is wired in this PR — that's intentional. The contract is the deliverable.

Part 2 — Shared observer skeleton extraction

• New backend/internal/observe package (one-paragraph package doc on top of observer.go) carries the observer-pattern-general pieces:
  • StartPollLoop(ctx, tick, poll, logger, name) — immediate first poll + ticker + ctx-done exit. SCM Observer.Start now delegates to it.
  • CredentialProbe + CheckCredentialsOnce(ctx, probe, checked, disabled, logger, name) — lazy first-poll credential gate. The SCM observer keeps credentialsChecked/disabled as Observer fields; the shared helper mutates them via pointer so state ownership stays single-source.
  • CacheSet[V any](m, order, max, key, value) + CacheDelete[V any](m, order, key) — one generic bounded-FIFO helper collapses the three near-identical cacheSet{String,Time,Bool} bodies and the standalone evictStrings. The SCM-side methods are kept as one-line wrappers that thread o.Cache.max through, so call sites and tests are untouched.
• backend/internal/observe/observer_test.go adds 11 small unit tests for the shared package, including a CacheSet-over-time.Time case to exercise the generic.

SCM behavior is unchanged. Both the existing 21-test SCM suite and the end-to-end test added in PR #115 stay green without modification.

Acceptance checklist (from issue body)

• ports.TrackerObservation + TrackerChanged exist with field-level documentation matching the SCM DTO style.
• lifecycle.Manager.ApplyTrackerFacts exists as a working reducer with the three initial reactions covered by unit tests.
• Shared observer helpers (goroutine supervisor, credential gate, bounded FIFO cache) live in a shared package and the SCM observer consumes them. SCM 21-test suite stays green.
• go build ./... && go test -race ./... clean (585 tests, 39 packages).

Validation

cd backend && go build ./... && go test -race ./...

→ 585 tests pass across 39 packages.

🤖 Generated with Claude Code

[greptile-apps]

Greptile Summary

This PR lands two scaffolding pieces ahead of the Tracker observer (#35): the ports.TrackerObservation DTO and the lifecycle.Manager.ApplyTrackerFacts reducer, plus extraction of shared observer primitives (StartPollLoop, CheckCredentialsOnce, CacheSet/CacheDelete) into a new observe package consumed by the SCM observer.

• ApplyTrackerFacts mirrors ApplyPRObservation's structure — Fetched gate → terminal-state shortcut (MarkTerminated) → session-read → ActivityWaitingInput/terminated guards → per-bucket reactions (log-only assignee, bot-comment dedup nudge via sendOnce with empty prURL for in-memory-only dedup). Two bugs found in review (CheckCredentialsOnce disabled short-circuit returning wrong value; bot-comment && vs || filter) were fixed before the review completed, and regression tests for both were added.
• Shared observe package collapses three near-identical cacheSet* bodies and the loop/checkCredentials duplication; newCache always normalises max to DefaultCacheMax (512), so CacheSet's maxEntries <= 0 guard is unreachable through the normal API and introduces no behavioural change in the SCM path.

Confidence Score: 5/5

Safe to merge — all changed paths are either new scaffolding with no live callers yet, or refactors of the SCM observer that preserve identical runtime behaviour.

The two bugs surfaced in earlier review rounds were both fixed and covered by regression tests before this review. The SCM refactor is behaviourally neutral. No new issues were found.

No files require special attention.

Important Files Changed

Filename	Overview
backend/internal/ports/tracker_observations.go	New provider-neutral TrackerObservation DTO mirroring SCMObservation; field-level docs are thorough and the shape is consistent with the existing contract.
backend/internal/lifecycle/reactions.go	Adds ApplyTrackerFacts reducer with terminal-state termination, assignee-log, and bot-nudge paths; dedup filter now correctly uses
backend/internal/lifecycle/manager_test.go	Eight new unit tests cover all reaction paths including the empty-ID regression test added to address a prior review finding.
backend/internal/observe/observer.go	New shared observe package extracting StartPollLoop, CheckCredentialsOnce (disabled short-circuit now returns !*disabled), and generic CacheSet/CacheDelete helpers.
backend/internal/observe/observer_test.go	11 unit tests covering cache, credential gate (including regression test for disabled short-circuit), and poll-loop; both spin-wait loops correctly capture deadline before the loop body.
backend/internal/observe/scm/observer.go	SCM observer loop, checkCredentials, and three cacheSet* methods collapsed to one-line wrappers; behaviour unchanged since newCache normalises max to DefaultCacheMax (512).

_{Reviews (3): Last reviewed commit: "test(observe): capture deadline once in ..." | Re-trigger Greptile}

[greptile-apps]

harshitsinghbhandari has reached the 50-review limit for trial accounts. To continue receiving code reviews, upgrade your plan.