Remove hardcoded TASK GUIDANCE; default 'code' task type owns it instead

[kylecarbonneau]

Remove hardcoded TASK GUIDANCE; default 'code' task type owns it instead

What

Removes the hardcoded buildSystemInstructions() "TASK GUIDANCE" block from the
executors and folds its content into the default code task type's
instructions instead.

Concretely:

• Deletes buildSystemInstructions() and every callsite that consumed it. That
  block was injected into each task launch through several channels:

  • Claude (claude_executor.go + the interactive launch paths in executor.go):
    a --append-system-prompt "$(cat <tmpfile>)" flag.
  • Gemini (gemini_executor.go): written to .gemini/GEMINI.md via
    writeGeminiInstructions() (also removed).
  • pi / codex / openclaw: appended to the prompt text.
    All of these are now gone, along with the dead temp-file creation and the
    per-launch flag. Each fmt.Sprintf that built a launch command was rebalanced so
    its verb count matches its argument count (the go vet printf analyzer stays
    clean).

• Moves the guidance into the default code task type's seeded instructions
  in internal/db/sqlite.go, matching the plain-text style already used by the
  writing and thinking defaults (and the existing code default). The content
  preserved: the taskyou_get_project_context MCP caching nudge, the worktree
  working-directory constraint, the taskyou_screenshot hint for frontend work, and
  the GitHub-CLI / shared-GraphQL-bucket etiquette.

• Adds a content-match-guarded migration for existing installs
  (migrateCodeTaskTypeGuidance): it runs
  UPDATE task_types SET instructions = <new> WHERE name = 'code' AND instructions = <old>,
  where <old> is the byte-exact previous default. It only upgrades a row that still
  holds the old default, so any user-customized code row is left untouched. It is
  idempotent — on the new schema the WHERE clause matches nothing.

Why

The TASK GUIDANCE block was opinionated, Claude-specific
(taskyou_get_project_context, worktree warnings, gh rate-limit etiquette), and
not user-controllable — there was no way to customize or opt out of it. It read
as a vestige of a single-team / single-workflow setup.

This change completes a direction already established in the codebase. In
commit cb7a3e1f ("Add PR submission reminder to coding task system prompt", branch
task/142-coding-tasks-system-prompt, merged via #65), the PR-submission reminder
for coding tasks was added to the code task type's instructions — the
user-prompt path — rather than to the executor's hardcoded block. That set the
precedent that coding guidance belongs in the task type, where users can see and
edit it. This PR follows that same path for the remaining executor-injected
guidance, consolidating all of it into the one place Bruno already chose.

Result:

• Default-config users get the same guidance, just delivered through the task type.
• Users (and custom task types) can now customize or disable that guidance via the
  existing task-type editing UI — exactly how the writing and thinking types are
  configured today.
• One fewer hardcoded Claude-ism in the executor; the guidance is no longer special
  cased per executor backend.

Note on delivery channel

The task-type instructions are delivered via the initial user prompt, whereas
the removed block was delivered via --append-system-prompt (Claude) /
GEMINI.md (Gemini). This is a slight delivery shift. For the default code type
the net effect on agent behaviour is indistinguishable — the agent receives the
same text either way. For users who care about the distinction, they now have full
control over the content through the task type, on whichever delivery channel the
backend uses for instructions.

How tested

• go build ./... — clean.
• go vet ./... — clean (the printf analyzer confirms every rebalanced launch
  Sprintf has matching verb/arg counts).
• go test ./... — all packages pass.
• New internal/db tests:
  • TestDefaultCodeTaskTypeGuidance — a freshly migrated DB seeds the code task
    type with the GitHub-CLI etiquette and the taskyou_get_project_context nudge.
  • TestMigrateCodeTaskTypeGuidance — a row holding the old default is upgraded;
    a customized row is left untouched (non-clobbering / idempotent).
• Removed the obsolete TestBuildSystemInstructions_GitHubGuidance (the function it
  asserted no longer exists); its intent is now covered by the db tests above.

Follow-up (separate PR)

A natural next step is to let users drop the default task-type guidance entirely
(remove the seeded instructions content for code/writing/thinking). That is
a larger behavioural change deserving its own migration and deprecation path, so it
is intentionally not bundled here.