feat(project): author the planning-suite mission for gh-plumbing (#262)

[nolte]

Summary

Authors the project/ planning suite for gh-plumbing so it ships a valid project/mission.md, closing the mission-authoring half of the portfolio backfill tracked in nolte/claude-shared#262 (gh-plumbing is one of the two prioritised W2 repos). The manifest (project/portfolio.yml) already exists; this PR adds the missing planning artefacts the mission spec (spec/project/mission/) requires as hard preconditions.

Because gh-plumbing is a mature, portfolio-wide configuration source whose three capabilities were already status: active before the planning suite was adopted, the MVP is modelled retroactively: roadmap items and the sprint/feature are recorded as done/closed to document the delivered surface, and mvp_status opens at achieved. This mirrors the retroactive-reconciliation pattern already used in nolte/kamerplanter's sprint 0001.

Changes

• project/goals.md — Vision (from the README intro) + four Outcomes (O-1..O-4) derived 1:1 from the three portfolio.yml capabilities plus the maintainer audience.
• project/roadmap.md — R-1..R-3 (reusable workflows / Probot commons / Renovate presets), each mvp: true, detail: fine, target_sprint: 1, status: done.
• project/sprints/0001-shared-cicd-baseline.md — retroactive sprint 1 (status: closed) with the consumer-facing value_statement.
• project/features/reusable-workflow-adoption.md — F-1, the mission-verifying feature (verifies_sprint_value: acceptance-1).
• project/mission.md — SMART mission with the eight required frontmatter fields, the four required sections, a per-audience paragraph for each of the four audiences, and verifies_via: F-1:acceptance-1.

All content is grounded in existing repo artefacts (README.md, project/portfolio.yml, AUDIENCES.md) — no invented audiences or missions, per the nolte/claude-shared#262 constraint.

Linked issues

Part of nolte/claude-shared#262 (mission-authoring backfill).

Testing

Structural conformance authored against spec/project/mission/, spec/project/roadmap/, spec/project/sprint/, and spec/project/feature/; the frontmatter field order, section order, audience↔paragraph bidirectionality, and verifies_via → feature/acceptance resolution were checked by hand against the specs. No CI planning-suite validator runs in this repo.

Risk / rollout notes

Additive project/ documentation only; no workflow, config, or published-surface change. Opened as draft for review of the retroactive-MVP modelling before it is applied as the template for the remaining nolte/claude-shared#262 repos. Trivially reversible (delete the five files).