CL-9007: ship shared summary-plus-small-tail pipeline for automatic and manual compaction

[TheGreatAxios]

Shared budgeted-tail pipeline for automatic and manual compaction: both funnel through capabilities.compact(pruning-compactor), so one pipeline change covers both.

What changes:

• Folds keep a structured handoff summary plus a small live tail (~7500-token default budget, ~5-10k tokens) instead of the last-N-verbatim keep window. keepRecentTurns stays as the legacy floor only.
• Tail tool outputs ride head/tail excerpted with a visible sentinel, original/kept lengths; full bytes stay stored (archive blob / adopted handoff file) and are never rewritten by excerpting. Excerpting is idempotent across repeat folds.
• When the keep-set covers everything foldable, apply still emits those excerpted live copies rather than the unexcerpted turns, so a short-but-bulky first compact does not keep the occupancy that armed it. No empty summary is invented.
• Re-excerpt skip matches the structured [tail-shortened N→ marker, not a raw prefix substring.
• Cuts never split a tool pair (whole-or-nothing partner drag); newest user messages with attachments stay whole up to the budget.
• Repeat folds pass the prior spine as priorSummary (update, don't restack) and carry the live tail forward; prior spines fold into the next handoff. Spine goal falls back to the fold narrative (minus the verify-repair appendix) when no user text was folded.
• record carries parameters.compactionShape plus decisions tailBudgetTokens/tailTokenEstimate/shortenedToolOutputs for the CL-9006 governor.
• Result-carrying turns no longer fuse into neighboring text turns, so excerpts stay independently budgetable; the budget-swallow early return keeps eager image aging (turns + spill blobs).
• SessionPruningCompactorArgs gains an optional compactionShape override (production default unchanged); integration harness forwards a test override.

Not in scope (per brief): governor thresholds/arming untouched, no per-family tuning, no archive storage migration, no second compactor.

Verification:

• bun run typecheck exit 0
• Compaction suites: 253 pass, 0 fail
• bun test ./src ./tests ./evals ./scripts --randomize --seed 424242 -> 8037 pass, 0 fail
• bun run lint -> 0 warnings, 0 errors
• Governor seam suite (src/agent/compaction.test.ts) shows no behavior change

Fixes CL-9007

[linear-code]

CL-9007