fix(arc-0d80.06mq9p): server-authoritative CWD resolution for git worktrees

[bfirestone]

Summary

• Fixes arc-0d80.06mq9p: arc ai session start --stdin (Claude Code SessionStart hook) failed with CWD ... does not resolve to a known project when invoked from a git worktree.
• Centralizes path-to-project resolution on the server side. CLI client now makes a single HTTP call instead of running its own 4-stage fallback chain.
• Net: 13 commits, +796/-131 across 15 files. New internal/gitfs package (pure-Go worktree detection, no git binary or third-party library at runtime). New internal/testutil/gittest shared test helpers.

Root cause

The CLI's resolveFromServer had 4 fallback stages (exact / normalized / git-main-worktree / subdirectory). The server's createAISession revalidated CWDs using only stage 1 (exact match). So a worktree CWD would resolve client-side via detectGitMainWorktree, then get rejected when the client POSTed the raw worktree path back to the server.

Approach

Server becomes authoritative for path resolution (consistent with the existing workspace_paths.hostname column that already implies local-per-host scoping):

1. New internal/gitfs package — pure-Go parser for .git pointer files. Handles normal repos AND bare-repo worktrees. ~30 lines of runtime code, no dependencies.
2. Broadened (storage.Storage).ResolveProjectByPath from exact-match to component-wise longest-prefix match, with literal _/%/\ characters in registered paths properly escaped via SQL REPLACE (ESCAPE '\\' alone wasn't sufficient — w.path is interpolated, not bound).
3. New (*Server).resolveProjectForPath helper — two-stage resolver (storage prefix match, then gitfs.DetectMainRepo fallback for worktrees outside any registered prefix).
4. Migrated createAISession and GET /projects/resolve to the new helper.
5. Collapsed CLI's 4-stage resolveFromServer to a single server call. Deleted ~88 lines of now-redundant client-side resolution logic.

Notable details

• ResolveProjectByPath is hand-written (not sqlc) because sqlc 1.30.0 cannot parse SQLite's ESCAPE clause. Documented precedent in CLAUDE.md (ListIssues, GetLabelsForIssues).
• The ESCAPE handling escapes w.path (the column), not the bound input — the metacharacter source is the registered path, not user input. Without this, a registered path like /home/john_doe/proj would falsely match /home/johnXdoe/projXfile.
• Bare-repo worktrees are supported — DetectMainRepo recognizes both <workdir>/.git/worktrees/<name> and <repo>.git/worktrees/<name> layouts. Validates the latter by checking for HEAD and objects/.
• Server is assumed local-per-host for path-aware features. Anyone running ARC_SERVER=https://remote/ for path-related features was already broken by the existing hostname column scoping; this PR doesn't change that, but documents it via behavior.

Test plan

• Unit: internal/gitfs — 11 tests including bare-repo worktree, malformed .git, relative gitdir, deep subdirectory walk
• Unit: internal/storage/sqlite/workspaces — 6 tests including longest-prefix-wins, component-boundary rejection, AND negative assertions that _/% in registered paths do NOT act as wildcards (lock the ESCAPE protection)
• Integration: internal/api/ai_sessions_test.go — TestCreateAISession_LinkedWorktreeOutsideRegistered does real git init + git worktree add and verifies end-to-end resolution. TestCreateAISession_RejectCrossProject confirms cross-project drift is still rejected (422).
• Integration: internal/api/workspace_paths_test.go — TestResolveProject_GitWorktree exercises the public REST endpoint against a real linked worktree.
• Full suite (make test) green.
• Runtime smoke: server restart with new binary required, then verify arc ai session start --stdin from any worktree no longer emits the old error. Tests prove the fix in code; activation is a deployment step.

Adversarial review pass

After the initial epic landed, an adversarial code review caught two real bugs and one coverage gap that the orthodox per-task review missed:

• The original ESCAPE '\\' clause was performative — it declared \\ as the pattern escape character but didn't actually escape _/%/\\ in the column value. Fixed via SQL REPLACE chain on w.path. New negative-assertion tests lock it down.
• DetectMainRepo silently failed for bare-repo worktrees. Now supported.
• Three near-identical git-test helpers lived in three files with inconsistent CI hardening (GIT_CONFIG_NOSYSTEM/GIT_CONFIG_GLOBAL set in two of three). Consolidated into internal/testutil/gittest.

T1's history was squashed (5 commits → 1) to remove a noisy buggy/revert/reapply sandwich; backup branch retained locally.

Files

cmd/arc/ai.go                                       (-1 / +1)
cmd/arc/main.go                                     (-94 / +6)
internal/api/ai_sessions.go                         (-1 / +1)
internal/api/ai_sessions_test.go                    (+92)
internal/api/resolver.go                            (+35)
internal/api/workspace_paths.go                     (-1 / +1)
internal/api/workspace_paths_test.go                (+45)
internal/gitfs/worktree.go                          (+109)
internal/gitfs/worktree_test.go                     (+165)
internal/storage/sqlite/db/queries/workspaces.sql   (-5)
internal/storage/sqlite/db/workspaces.sql.go        (-24)
internal/storage/sqlite/workspaces.go               (-4 / +35)
internal/storage/sqlite/workspaces_test.go          (+185)
internal/testutil/gittest/gittest.go                (+63)
internal/testutil/gittest/gittest_test.go           (+59)

Closes arc-0d80.06mq9p.