fix(build): strip Windows verbatim \?\ prefix from registry common_dir

[coseto6125]

Problem

On Windows, ecp <cmd> --repo <name> fails with:

Error preparing index for <repo>: build_l2: 目錄名稱無效。(os error 267)

os error 267 is ERROR_DIRECTORY / ENOTDIR. The same command via cwd
(cd into the repo, no --repo) works fine — so it is not a CJK path, not a
corrupt index, and not a registry-corruption issue (doctor reports the
registry clean).

Root cause

std::fs::canonicalize returns a verbatim path on Windows
(\\?\C:\...). git_common_dir_string (build path) stored that verbatim
string into the registry's common_dir:

"common_dir": "\\\\?\\C:\\Revice_Code\\backstage_api_test_new\\.git"

Resolving --repo <name> derives worktree_root by taking the parent of
common_dir and hands it to ensure_fresh / build_l2. The verbatim prefix
flows into ignore::WalkBuilder and git archive, which treat the .git
file component as a directory and bail out with ERROR_DIRECTORY (267).
The cwd path never hit this because its worktree root comes from
current_dir — a plain path.

Fix

Two points, so the bug is both prevented and recoverable without a rebuild:

Layer	Change	Why
build/orchestrator.rs	write common_dir via dunce::canonicalize	root cause — new registry entries are plain
git_cache.rs	worktree_root_from_common_dir runs dunce::simplified	defensive — registries already holding a verbatim prefix resolve correctly without re-indexing

update_repo_meta only writes common_dir on first build, so existing repos
keep their old verbatim value — the read-side simplified is what unblocks
those without forcing a rebuild. Both dunce calls are no-ops on non-Windows
and on already-plain paths (simplified returns a borrowed sub-slice, zero
alloc).

Deliberately not touched: head_sha_hex's std::fs::canonicalize
(orchestrator.rs:368) only feeds an xxhash digest for cache identity — it
never reaches the build pipeline, and it must stay byte-identical to the
matching digest in git_cache.rs so existing non-git caches remain valid.

Tests

• worktree_root_drops_dot_git_component, worktree_root_falls_back_when_no_parent — cross-platform.
• worktree_root_strips_verbatim_disk_prefix, worktree_root_leaves_plain_path_untouched — #[cfg(windows)].

cargo test -p egent-code-plexus green; repo_selector (18) and
build_orchestrator suites unaffected.

🤖 Generated with Claude Code

[coseto6125]

Unified Code Review — PR #528

Risk: MEDIUM (Windows-only; build pipeline + pre_tool_use hot path touched, but change is a zero-alloc borrowed-slice transform)
Affected Flows: ensure_fresh → build_l2; find / group impact / pre_tool_use query paths

Root cause analysis is correct and independently confirmed: std::fs::canonicalize emits a \\?\ verbatim path on Windows; stored into registry common_dir, it flows into ignore::WalkBuilder / git archive which treat the .git file component as a directory → ERROR_DIRECTORY (267). The two-point fix (write-side dunce::canonicalize, read-side dunce::simplified) is the right shape, and dunce::simplified correctly preserves non-strippable verbatim paths (\\?\UNC\, \\.\, reserved names) — verified against dunce 1.0.5 is_safe_to_strip_unc source + its test suite. Compiles clean, cross-platform tests green.

Found 1 issue:

1. Fix is incomplete — three sibling paths still hit os error 267 (Score: 80) — Bug
   The PR fixes worktree_root_from_common_dir, but three call sites derive the worktree root by open-coding common_dir.parent() instead of going through that (now-fixed) helper, so a registry holding a verbatim common_dir still feeds a verbatim path into a walk / git subprocess on Windows:

   • egent-code-plexus/crates/ecp-cli/src/commands/summary.rs

     Lines 176 to 177 in c99bb08

     	let common = Path::new(&r.common_dir);
     	let worktree = common.parent().unwrap_or(common);

     → ensure_index → any_source_newer_than → WalkBuilder::new(root) (ecp summary)

   • egent-code-plexus/crates/ecp-cli/src/commands/group/sync.rs

     Lines 167 to 171 in c99bb08

     	let common_dir = std::path::PathBuf::from(&alias.common_dir);
     	let src_root = common_dir
     	.parent()
     	.map(|p| p.to_path_buf())
     	.unwrap_or(common_dir.clone());

     → walk_and_extract → WalkDir::new(src_root) (ecp group sync)

   • egent-code-plexus/crates/ecp-cli/src/commands/group/status.rs

     Lines 94 to 96 in c99bb08

     	let common_dir = std::path::PathBuf::from(&alias.common_dir);
     	let repo_root = match common_dir.parent() {
     	Some(p) => p.to_path_buf(),

     → git -C repo_root — verbatim rejected by git's SetCurrentDirectory (ecp group status)

   Same root cause, same fix: replace each open-coded .parent() with crate::git_cache::worktree_root_from_common_dir(common_dir). Recommend bundling into this PR since they share the fix and are otherwise a guaranteed repeat 267 the moment a Windows user runs summary / group.

Nit (non-blocking): PR body carries a Generated with Claude Code footer — this repo's CLAUDE.md forbids that trailer on PR bodies; drop it on the next push.

[github-actions]

ecp impact cache (0 symbols) — internal, used by ecp dev pr-analyze

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