fix(pre_seed): security hardening + tempdir permission fix

[chaodu-agent]

Fixes

#	Severity	Fix
F1	🔴	Symlink follow in permission modification — use symlink_metadata() instead of is_file() to prevent symlink-following attacks when setting permissions
F3	🟡	Target dir not created — create_dir_all(target) before tempdir_in(target) to avoid permission denied when target doesn't exist
F4	🟡	Symlinks followed in move_recursive — use symlink_metadata() and preserve symlinks as-is without following (prevents directory escape)
F5	🟡	Docs mismatch — update config-reference.md and hooks.md to reflect pre-seed is now default-on
—	—	Clippy — replace % N == 0 with .is_multiple_of(N) (Rust 1.96 lint)

Not Fixed (already safe)

F2 (path traversal): tar::Entry::unpack_in() already rejects .. components and absolute paths — no additional validation needed.

[chaodu-agent]

LGTM ✅ — Security hardening is solid, all critical findings from prior round have been addressed.

What This PR Does

Hardens the pre_seed module against symlink-following attacks, fixes a tempdir creation failure when target directory does not yet exist, updates docs to reflect pre-seed is now default-on, and applies a Clippy lint fix.

How It Works

• Uses symlink_metadata() instead of is_file() / is_dir() to avoid following symlinks when setting permissions and during recursive move
• Adds create_dir_all(target) before tempdir_in(target) to ensure the target exists and keeps temp files on the same filesystem
• Introduces create_symlink_or_copy() to preserve symlinks on Unix; rejects escaping targets (absolute/..); errors on non-Unix when resolved target is missing
• move_recursive symlink branch correctly handles existing destinations (dir vs file) before replacing
• Updates docs (config-reference.md, hooks.md) to remove stale feature-flag language

Findings

#	Severity	Finding	Location
1	🟢	create_dir_all(target) + tempdir_in(target) correctly fixes parent-not-writable case, keeps extraction on same filesystem	pre_seed.rs:175-176
2	🟢	symlink_metadata() in tarball extraction prevents chmod on symlink targets	pre_seed.rs:310
3	🟢	move_recursive symlink branch correctly distinguishes dir vs file via symlink_metadata, uses remove_dir_all vs remove_file	pre_seed.rs:389-408
4	🟢	create_symlink_or_copy rejects escaping targets (absolute/..) and errors on non-Unix when resolved target missing — no silent data loss	pre_seed.rs:343-378
5	🟢	Tarball symlink escaping: absolute/.. targets removed + warned	pre_seed.rs:302-321
6	🟢	Zip permissions strip suid/sgid/sticky (& 0o0777)	pre_seed.rs:259
7	🟢	.is_multiple_of(N) — clean Clippy fix	pre_seed.rs:225,289
8	🟢	4 Unix tests cover: symlink preservation, symlink overwriting existing dir, tempdir with non-writable parent (regression), deep target creation	pre_seed.rs tests
9	🟢	Docs update for pre-seed default-on is accurate for binary consumers	docs/
10	🟡	Docs say "pre-seed is enabled by default" — true for the binary but openab-core crate still has it as opt-in feature. Consider noting this for library consumers	config-reference.md:248, hooks.md:21

Finding Details

🟡 F10: Docs slightly inaccurate for crate consumers

The docs state "pre-seed is enabled by default. No feature flag needed." This is accurate for the OpenAB binary (which enables the feature), but library consumers of openab-core still need features = ["pre-seed"]. Consider adding a note for library users. Non-blocking — the docs target end-users of the binary.

Baseline Check

• PR opened: 2026-06-25
• Author: chaodu-agent (self-fix PR addressing prior review findings)
• Main already has: pre_seed.rs with tarball/zip extraction, move_recursive, permission setting
• Net-new value: symlink attack hardening, tempdir placement fix, regression tests, docs accuracy

What's Good (🟢)

• Comprehensive symlink defense: metadata-based detection, escaping target rejection, proper cleanup
• Same-filesystem tempdir avoids cross-device rename overhead
• Test coverage for all new code paths including edge cases
• Clean separation of Unix vs non-Unix behavior in create_symlink_or_copy
• Defensive create_dir_all(target) ensures robustness in constrained environments