feat(traceability): append SF-Session id to autonomous commit messages

- git-service.js autoCommit() accepts optional sessionId param
  - Appends 'SF-Session: <id>' trailer to commit message when present
  - Falls through cleanly when sessionId is undefined (quick tasks, templates)
- worktree.js autoCommitCurrentBranch() forwards sessionId
- auto-post-unit.js autoCommitUnit() reads session ID from getAutoSession()
  via s.cmdCtx?.sessionManager?.getSessionId?.() — same pattern as auto.js

Mirrors Copilot's session logs linked to each commit for cross-session traceability.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/resources/extensions/sf/auto-post-unit.js

@@ -165,6 +165,7 @@ import { join } from "node:path";
 import { describeNextUnit } from "./auto-dashboard.js";
 import { _resetHasChangesCache } from "./native-git-bridge.js";
 import { autoCommitCurrentBranch } from "./worktree.js";
+import { getAutoSession } from "./auto/session.js";
 
 /**
  * Detect summary files written directly to disk without the LLM calling
@@ -336,11 +337,13 @@ export async function autoCommitUnit(basePath, unitType, unitId, ctx) {
 		if (LIFECYCLE_ONLY_UNITS.has(unitType)) {
 			return null;
 		}
+		const sessionId = getAutoSession().cmdCtx?.sessionManager?.getSessionId?.() ?? null;
 		const commitMsg = autoCommitCurrentBranch(
 			basePath,
 			unitType,
 			unitId,
 			taskContext,
+			sessionId,
 		);
 		if (commitMsg) {
 			ctx?.ui.notify(`Committed: ${commitMsg.split("\n")[0]}`, "info");

src/resources/extensions/sf/git-service.js

@@ -504,7 +504,7 @@ export class GitServiceImpl {
 	 * Returns the commit message on success, or null if nothing to commit.
 	 * @param extraExclusions Additional paths to exclude from staging (e.g. [".sf/"] for pre-switch commits).
 	 */
-	autoCommit(unitType, unitId, extraExclusions = [], taskContext) {
+	autoCommit(unitType, unitId, extraExclusions = [], taskContext, sessionId) {
 		// Quick check: is there anything dirty at all?
 		// Native path uses libgit2 (single syscall), fallback spawns git.
 		if (!nativeHasChanges(this.basePath)) return null;
@@ -512,9 +512,10 @@ export class GitServiceImpl {
 		// After smart staging, check if anything was actually staged
 		// (all changes might have been runtime files that got excluded)
 		if (!nativeHasStagedChanges(this.basePath)) return null;
-		const message = taskContext
+		const base = taskContext
 			? buildTaskCommitMessage(taskContext)
 			: `chore: auto-commit after ${unitType}\n\nSF-Unit: ${unitId}`;
+		const message = sessionId ? `${base}\nSF-Session: ${sessionId}` : base;
 		nativeCommit(this.basePath, message, { allowEmpty: false });
 		// Absorb any preceding sf snapshot commits into this real commit.
 		// Walk backwards from HEAD~1 counting consecutive snapshot subjects,

src/resources/extensions/sf/worktree.js

@@ -281,8 +281,9 @@ export function autoCommitCurrentBranch(
 	unitType,
 	unitId,
 	taskContext,
+	sessionId,
 ) {
-	return getService(basePath).autoCommit(unitType, unitId, [], taskContext);
+	return getService(basePath).autoCommit(unitType, unitId, [], taskContext, sessionId);
 }
 // ─── Git HEAD Resolution ────────────────────────────────────────────────────
 /**