refactor: extract fork/merge into a tape sidecar

[PsiACE]

Summary

Move fork/merge out of Tape and Agent into a builtin sidecar. Callers explicitly decide whether to merge or discard a fork.

Plain TapeStore
    └── Session Tape
          ├── spill sidecar
          └── forkmerge sidecar

plain tape
    └── fork()
          ├── merge()   → persist main and spill writes
          └── discard() → drop isolated writes

Behavior

• Normal and concurrent channel turns merge when their streams finish.
• Inherited and named subagents merge on success.
• Temporary and failed subagent runs are discarded.
• Inherited and temporary subagents can read the active parent context.

Verification

• Ruff
• mypy
• pytest: 300 passed
• Documentation build
• Manual runtime checks for bub run, subagent session modes, and concurrent Telegram turns

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	bub	0038d96	Commit Preview URL Branch Preview URL	Aug 24 2026, 02:47 PM