feat(engine): implementation runs follow the session binding

[hlubek]

Implements the first slice of 20260914-180822-d-cpt-9kv: the implementation procedure holds one branch fact, baseBranch; the work branch lives only in the session binding.

What changes

• Procedure (d-prc-imp): workBranch and the workTarget step are gone; setup routes straight to the working junction and tells the agent to declare the session binding once the host is on the work branch. Dispatch seeds carry no branch, so captures during work and the closing done follow the binding. The landing unit states the order: merge, return to base, clear the binding, then report landed. The closeout no longer speaks of the binding. The work junction gains an abandon path that removes the marker on base (unwind → unmark when a marker exists).
• Target rule: workBranch leaves the branch-field precedence registry, so the binding is the only branch fact after base; a stale value in an old session's store is ignored. A graph read that a state field sends to an unavailable branch now says so: reading the graph on branch "x", chosen by the captureBranch state field, failed: … instead of the bare mutation-target message.
• Replay: a session bound to a branch whose checkout is gone could neither be resumed nor have its binding cleared, because replay resolved procedure specs on the bound branch. Spec resolution falls back to the project's default branch in that case; the entry-identity check still guards the procedure. This is what lets the two runs stuck at landing today be unbound and finished.
• Tests: proctest worlds can drop a branch (World.DropBranch) the way the local runtime loses a checkout. New regressions: landing after the sole checkout returns to base (TestImplementation_LandsAfterTheSoleCheckoutReturnsToBase), the abandon path, and clearing a stale binding. The remaining implementation tests follow the new flow.

Already covered before this PR: the engine acquires no graph for a command that declares none (GraphIndependent, since 65aeca3).

Left for later slices

• The declared coding-host session property (start_session declaration, template exposure, gating of the implementation move).
• Remote branch delivery for a session host over an embedded graph.

Both stay open under d-cpt-9kv and will be named in the closing done.

🤖 Generated with Claude Code

Confidence Score: 5/5

The PR appears safe to merge; no outstanding previous finding or new actionable defect was identified.

Summary

This PR makes implementation runs follow the session’s effective branch rather than storing procedure-owned branch fields.

• Derives an unbound local session’s base from the serving checkout and exposes that source in session framing.
• Routes marker, capture, document, and done writes through the session binding or derived base.
• Makes replay tolerate retired fields and steps so older sessions can resume after procedure evolution.
• Removes the explicit landing junction, releasing the marker alongside the done entry so both travel with the work.
• Adds regression coverage for branch/worktree runs, stale bindings, removed checkouts, replay compatibility, and abandonment.
• Replaces the Astro documentation action with an explicit pinned pnpm build and Pages artifact upload.

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