Remove non-owner preflight, start and status fallbacks after N-2 #907

Problem

The preflight, start and status implementations together account for 445 code lines at packages/agents/source/skills/rebase/shared/scripts/preflight.sh:1, packages/agents/source/skills/rebase/shared/scripts/start-rebase.sh:1 and packages/agents/source/skills/rebase/shared/scripts/rebase-status.sh:1. Their Go replacements permit removal after the warning window. Removing all rebase shell at once would leave almost no room below 800 lines for forwarding paths or verification.

Part of #588 and #326.

Acceptance criteria

1. AC1: Given the fallback-removal release, when compatibility evidence is inspected, then the required binary and at least two warning releases precede removal.
2. AC2: Given the three assigned legacy entry points, when invoked with a supported binary after removal, then they forward through the shared launcher with their established arguments.
3. AC3: Given the resulting deployment, when implementation content is inspected, then the assigned legacy algorithms are absent.
4. AC4: Given frozen recovery evidence, when a remaining compatibility command and a migrated command exchange it, then the existing workflow resumes correctly.
5. AC5: Given the removed algorithms, when their coverage ledger is inspected, then their still-supported behavior has executing Go coverage.
6. AC6: Given the removal diff, when protected paths are classified, then no owner or constitutional path is changed or deleted.
7. AC7: Given regenerated packages and deployments, when consistency checks run, then they pass.
8. AC8: Non-functional: removal introduces no new shell logic or resolver.

Scope and non-goals

Scope: only non-owner preflight, start and stopped-status fallback algorithms, their generated copies, fixed forwarding entries using #321's shared launcher and Go integration adjustments.

Non-goal: Bats or Python test retirement, which must land separately in child 11. Non-goal: continue, verify and scanner fallbacks (child 14), conflict resolution (child 15), shared readiness (#322), shared libraries (#321), or any guard. Owner and constitutional deletions belong only to the single cutover under #588 with owner exact-head review. Retain both guard sources and every protected copy.

Approach

Use the ledger from child 11 and release evidence from child 10. Delete the three assigned non-owner implementations or their unchanged retained fallback locations if #321's delivery contract relocated them. Preserve public script names as fixed forwarding shims into the already-shared launcher; no local selection or version logic. Recount any copies outside the permitted generated directories. Preserve state interoperability while child 14's fallbacks remain. Regenerate from canonical sources, without editing generated artifacts by hand.

Dependencies

Blocked by: #895, #896, #897, #904, #905, #333, #544, #753.

Prerequisites: #321's launcher and the completed N-2 warning window. This child does not wait on the final cutover under #588; it hands protected deletions to that cutover and creates no cycle.

Size

M. Forecast 555 code lines: 445 measured implementation deletions, 30 fixed forwarding lines and 80 Go-integration/deployment adjustments. Count relocation changes as additions plus deletions; do not hide them as generated output. Prose allowance: 220 lines.

scc could not run in the isolated checkout. Measured baselines count non-blank lines other than full-line # comments, including Python docstrings and embedded code conservatively. Forecasts include implementation, tests, fixtures and registration; they are estimates, not measured future Go. Generated output is excluded only under packages/agents/claude/.apm/skills/, packages/agents/codex/.apm/skills/, .claude/skills/, .agents/skills/ and tokens.json. The source under packages/agents/source/ and copies anywhere else count. The final diff must be at most 800 code lines and 2,000 prose lines, with no exception; split before implementation if either forecast grows beyond its limit.

Verify plan

• AC1: inspect released versions and documented warning history before allowing removal.
• AC2, AC3, AC8: inspect canonical and generated entry contents and exercise each public invocation through the shared launcher.
• AC4: exercise mixed old/new preflight, start, status and continue/verify evidence transitions in isolated repositories.
• AC5: compare removed algorithms with child 11's ledger and executing Go CI results; confirm no Bats or Python test retirement is included.
• AC6: apply origin/main's owner and constitutional patterns to additions, modifications, renames and deletions.
• AC7: regenerate, refresh deployment records and run generated-consistency and packaging checks.
• Definition of done: D1 each AC is verified at the merged head, with the commands and results cited in the pull request's Verification. D2 an independent review approves the exact merged head, and every finding has a recorded disposition. D3 required checks pass with no gate loosened. D4 the leak scans the repository specifies are clean, and every post made for this work passes the per-post leak review before it is posted. D5 every commit is signed. D6 the documentation matches the change or the Verification says why there is no impact. D7 every commit has an Assisted-by trailer and the pull request has the provenance:agent-assisted label. D8 the pull request stays within the size limits. D9 follow-ups are filed as issues and listed under Related. D10 the merge closes the issue through a closing reference, the board shows Done, and the final Status: comment names the merge commit.

Risks

Removing before consumers can run the required binary strands legacy callers. Mixed state formats could lose the old comparison range. Undo: revert removal and regenerate the frozen fallbacks without adding logic.

Operator authority

None.