Describe deterministic stack replay

[nishu-builder]

Represent a feature as numbered source gitlinks and .base files recording where each layer started. A replay plan selects changes, messages, and output branch boundaries.

Each run starts from the beginning with fixed commit metadata. A conflict leaves a draft to edit; the next plan references that resolution. The same tool prepares new layers and restacks existing ones. Update the chat and GitHub guides to describe this workflow.

[nishu-builder]

Review for simplicity and parsimony, covering the design and how #279–#282 implement it.

The main point (inline at L139-152): since every replay step is a pure function of object ids, the replay can re-run the whole plan on each call instead of pausing and resuming. With a fixed committer, output commits are the same on every run, and the merges are cached jobs. That removes action=continue, the done/here records in the plan, the rules for what stays fixed while paused, the stored original-snapshot tree id, and the history walk in #282's rebase.rs::checkpoint. That is most of the 1390-line rebase_plan.rs and most of rebase_tests.rs. A conflict is resolved by editing the plan to reference the fixed draft.

Other reductions, each inline:

1. Plan commands: drop is a no-op, squash is a range pick plus a message, and amend is message. Four commands cover it: onto, pick, message, branch.
2. branch= takes bare names and the tool assigns numbers.
3. git-add-layer is a one-layer replay. Consider one writer, and dropping the special -work name.
4. Writers get the stack subtree as in, not the whole conversation. This removes the out-of-scope diff check and allows cache hits.
5. #279 adds a second merge-tree implementation (a server route) beside std/merge, and a git-commit-tree subcommand that duplicates caos put-commit.
6. design/chat.md and design/agent-github.md describe a different stack model (numbering from 01, restack by merge, no special names) and need updating in this PR.

---

Generated by Claude Code

[nishu-builder]

Closing as superseded: #293 put the stack design in design/stacks.md with a different model (sibling gitlinks ordered by name, kept current with merge, not .base files plus a replay plan), and #300 is rewriting design/agent-github.md.

---

Generated by Claude Code