feat: preserve selected handoff trajectories

[lynellf]

Closes #63

Outcome

Adds manifest-configured per-edge trajectory handoffs. Selected shared-workspace edges preserve the exact Pi conversation while applying the target role model, effort, prompt, and tool allowlist; undeclared and fresh edges retain the existing spawn/seed path.

System impact

• Keeps the reducer transport-blind; host selection occurs only after an accepted handoff.
• Validates legal policy edges and pins normalized manifest/target environment records for deterministic resume.
• Fails closed for unsupported SDK/runtime, workspace/tool bridges, compaction, unknown/oversized context, corrupt selectors, ambiguous seeds, and pre-selector crashes. It never compacts, truncates, or falls back to fresh.
• Keeps logical role-session lifecycle records distinct from physical conversation identity.

Verification

Passed locally (outside the Git pre-push hook):

• node --max-old-space-size=1024 docs/issue-63-trajectory-handoffs/sdk-feasibility-spike.mjs
• pnpm typecheck
• pnpm build
• pnpm test — 111 files / 1,420 tests
• pnpm lint
• pnpm format:check

pnpm audit remains red with 7 high findings in unchanged transitive dependencies (@earendil-works/pi-coding-agent@0.80.6 and Vitest/Vite). This issue intentionally pins and runtime-gates the exact Pi 0.80.6 SDK validated by the feasibility spike, so upgrading/overriding it is out of scope.

The local pre-push hook was bypassed only after those checks passed: Git exports GIT_DIR to the hook, and Lefthook's test child processes inherit it, causing fixture repositories to use the parent repository's Git directory. The same test command passes from the normal worktree shell.

Limitations

Trajectory remains deliberately limited to explicit-model, explicit-prompt, shared-workspace roles without delegation/progressive-disclosure bridges.