fix(coding-agent): expose failed compaction attempts

[minpeter]

Summary

• publish compaction_start once an automatic compaction owns its model/controller
• do so before authentication/preparation can fail
• preserve the one-overflow-retry attempt when preparation fails before a provider retry

This is a one-commit follow-up to merged PR #310, based directly on its merge result.

Root cause

The final #310 controller-supersession fix moved compaction_start after authentication and request preparation. An
overflow compaction that failed during those early preparation steps therefore consumed/released internal attempt
state without publishing the attempt. The next prompt could not prove that the overflow recovery had been retried,
and the established no-auth regression observed zero starts instead of two independent attempts.

The event now emits after model/controller ownership is proven but before fallible authentication/preparation. A
superseded controller still cannot emit; a valid owned attempt remains observable even when preparation returns early.

RED -> GREEN

• RED on exact merged main:
  • does not consume the overflow compact-and-retry attempt when compaction fails before retrying
  • expected two compaction_start events, received zero
• GREEN:
  • exact target 1/1
  • agent-session compaction + controller lifecycle group 42/42
  • full compaction/retry/hook matrix 14 files / 121 tests

Verification

• root build: passed
• root npm run check: passed
• real source CLI mock loop: exit 0, one localhost request, expected assistant text, auth unchanged
• built CLI --help: exit 0
• built CLI invalid install input: exit 1 with usage

The broader coding-agent full suite has unrelated load-sensitive MCP/app-server/temporary-filesystem failures which
pass directly; no compaction test remains failing.

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Summary by cubic

Emit compaction_start as soon as an automatic compaction owns its model/controller, before auth and preparation. This makes early preparation failures visible and preserves the single overflow retry attempt.

^{Written for commit 62354c2. Summary will update on new commits.}