feat: Move-Commit -UnderPRInStack (stack a commit under an existing PR)

[dkattan]

Summary

Adds a -UnderPRInStack parameter set to Move-Commit that automates the real-world workflow: "I have a commit in this PR I don't want bloating it — move that code to another branch/worktree/PR stacked UNDER this one, control draft status, then spawn another agent in that worktree."

Non-destructive by default: the source branch is never rewritten or force-pushed. The moved commit leaves the source PR's diff purely because the diff becomes lower..source (which excludes it). -RemoveFromSource remains the opt-in rewrite path.

What it does

Cherry-picks the commit onto a new lower branch (in a persistent, user-owned -WorktreePath handed to a second agent), pushes it, opens a draft PR, repoints the source PR's base to the lower branch, and registers the GitHub-native stack.

Move-Commit -UnderPRInStack -WorktreePath <path-to-persistent-worktree>
# zero extra args: title derives from the commit message, source PR derives from the branch

Architecture: a new recorder plan core

The existing New-MoveCommitPlan builds ~250 lines of PowerShell as string literals — hard to extend to a flow that's 60% live gh side-effects. This PR adds a recorder plan core where ops are data nodes with two interpreters (execute-live + serialize-to-script), so strings become an output of serialization, never an input. This is the tagless-final/command pattern (precedented in theory via Darcs, novel in git tooling).

• Additive only — Move-Commit's existing behavior is untouched (DefaultParameterSetName='Move'). The recorder is used by the new parameter set only; retrofitting Move-Commit is a deferred follow-up.
• Narrow DSL by design: per-op OnFailure, one finally, When guards, Down rollback companions, $runtime: token capture. No general control-flow AST.

Stack registration: gh stack link, not gh stack submit

Source-level verification of gh-stack v0.0.8 showed gh stack submit --auto fails in this flow (requires local .git/gh-stack tracking state we never create) and would re-touch the base from a divergent local model. gh stack link <lower-pr> <source-pr> needs no local state, computes bases from argument order, reuses existing PRs, and is a no-op confirmation when the base is already correct.

Idempotent re-run + rollback (D8)

Because the default is COPY (no source rewrite), the whole flow is idempotently re-runnable — primary recovery is "re-run the command." Pre-flight fail-fast checks and per-step "already done?" resume checks are one mechanism used twice. Each op carries a Down (undo) companion; on failure it reports the exact rollback commands by default, and -RollbackOnFailure auto-plays them. The cherry-pick step uses git patch-id --stable to skip when the patch is already applied (sanity-checked against real git).

Test plan

• 285 Pester tests pass (243 original + 42 new), no regressions, zero PSScriptAnalyzer errors.
• New tests cover: recorder op model, builders-that-don't-execute, both interpreters, $runtime: capture; gh/stacks runners, branch→PR resolution, stale-stack discovery, provider dispatch; the under-stack flow's parameter set, zero-arg ergonomics, the persistent-worktree guardrail (no worktree remove), patch-id idempotency, 3-way worktree detection, D5 stale-unstack ordering, D7 draft/title, and D8 failure reporting + rollback buffer.
• Not yet run: live end-to-end against a real repo with stacks (immybot). Unit/snapshot/mock coverage is comprehensive; the live e2e is a follow-up before merge.

Design provenance

The 9 design decisions (D1–D9) were settled via the Wayfinder approach — charted as tickets, resolved one at a time, with the two load-bearing primitives (git patch-id --stable, git worktree list --porcelain) sanity-checked against real git before encoding. Key decisions recorded in repo memory.

Follow-ups (deferred by design)

• Retrofit Move-Commit/New-MoveCommitPlan onto the recorder.
• Persisted phase-tracked plan with --continue/--abort (only if transient failures prove frequent in practice).
• Live e2e against immybot before merge.

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