fix(#1526): delegate auto-chain post-completion to transition workflow (#3419)

* fix(#1526): delegate auto-chain post-completion to transition workflow

execute-phase's auto-chain completion called phase.complete then a light inline
set (partial PROJECT.md update + offer-next) and never invoked the transition
workflow, silently skipping graduation scan, session-continuity, project-reference,
accumulated-context, and current-position updates — so a phase completed via
auto-chain left different project state than a normal transition.

Fix (delegate, user decision 2026-08-13): replace execute-phase's update_project_md
+ offer_next with a delegation step that @-includes transition.md in post-completion
mode. Add a post_completion_mode step to transition.md that skips verify_completion
+ update_roadmap_and_state (phase.complete already ran; avoids double-write) and
begins at evolve_project. Standalone transition (mode 1) is unchanged.

Regression: tests/auto-chain-transition-delegation.test.cjs (source-text-is-the-
product) asserts the delegation, the skip-set, the removed inline step, and the mode.
Ack fragment 1526 covers execute-phase.md + transition.md growth (spent 2930 fragment
removed — same-path owner conflict, like #3025/#3024).

* docs(#1526): backfill changeset PR number (#3419)

---------

Co-authored-by: sim <sim@local>

.changeset/lucky-ravens-leap.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 3419
+---
+**Auto-chain phase completion now runs the same post-processing as a normal transition** (#1526) — completing a phase via `/gsd:execute-phase` (auto-chain) previously skipped the transition workflow's graduation scan, session-continuity, project-reference, accumulated-context, and current-position updates, leaving project state different from a normal transition. execute-phase now delegates post-completion processing to the transition workflow (post-completion mode: skips re-verify + re-running `phase.complete` to avoid a double-write). Identity/standalone transition behavior is unchanged.