Assert tracked tree holds nothing repository walkers skip (#170)

[afflom]

Closes #170

Summary

• Extends RP-03 (SPEC.md §7 and §31, model/ids.toml, CONFORMANCE.md, features/suites/repository.feature, and crates/conformance/src/cases/repository.rs) to inspect every tracked path (git ls-files) and fail if any tracked path is named CACHEDIR.TAG or has a path component the repository walkers skip (target, .git, .lexlean, .lake, node_modules, or expected outside the sanctioned expected/ oracle directories under bootstrap/, compiler/, stdlib/, examples/<example>/, tests/negative/<case>/, tests/tooling/<suite>/<case>/, and tests/fixtures/<suite>/<case>/).
• Adds CACHEDIR.TAG directory skipping to the remaining three repository walkers (xtask/src/audit.rs, crates/conformance/src/lib.rs, and crates/conformance/src/cases/oracle.rs) so that every repository walker skips CACHEDIR.TAG directories.
• Records the planted-defect runs (a tracked examples/collections/CACHEDIR.TAG and a tracked crates/lexlean/.lexlean/planted.lean) in VERIFICATION.md and CHANGELOG.md.
• Touches no files in SEMANTIC_DIRS and keeps the conformance ID register at 404 IDs.

[afflom]

@afflom @humuhumu33 Ready for review.

• issue-170-tracked-walker-skips (e2f65937b) contains main (8765feea3).
• Draft rest leg, honesty, and reproducibility passed on e2f65937b.
• Extends RP-03 (keeping the register at 404 IDs and touching nothing in SEMANTIC_DIRS) to verify via git ls-files that no tracked path is named CACHEDIR.TAG or has a walker-skipped component outside sanctioned expected/ oracle directories, and adds CACHEDIR.TAG directory skipping to xtask/src/audit.rs, crates/conformance/src/lib.rs, and crates/conformance/src/cases/oracle.rs.
• Planted examples/collections/CACHEDIR.TAG and crates/lexlean/.lexlean/planted.lean failures are recorded in VERIFICATION.md.

[afflom]

@afflom @humuhumu33 Full ready-for-review gate run 39097582720 is 25/25 green (honesty and reproducibility also green: 27/27 checks passed) on e2f65937b, which contains main (8765feea3) and touches nothing in SEMANTIC_DIRS.

[afflom]

Rebased onto main (193cd2148) at 53eae08d3 and verified cargo xtask validate, conformance_rp_03, and conformance_rp_08 locally. cc @afflom @humuhumu33

[afflom]

@afflom @humuhumu33 Full ready-for-review gate run 39113716975 is 25/25 green (honesty and reproducibility also green: 27/27 checks passed, mergeStateStatus: CLEAN) on 53eae08d3, which contains main (193cd2148) and touches nothing in SEMANTIC_DIRS.

[afflom]

Landed through wave 2, #171, merged to main as d51d9f223: this branch's head 77553dbf3 (the #170 assertion on top of main) is merged into that PR unchanged, so its commit is in main's history and #170 is closed by the merge. Closing this PR by hand; its gate ran as part of #171's run 39125361480.

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