fix(tools/fossil, tools/vcs): detect Fossil checkouts by .fslckout

[dpol1]

Summary

• tools/vcs (FossilBackend.detect()) and tools/fossil (find_repo_db()) looked for a checkout file named .fslckg. Fossil never writes that name: its glossary says the checkout database "is in the .fslckout file on POSIX type systems, but for historical compatibility reasons, it's called _FOSSIL_ by native Windows builds of Fossil."
• On Linux and macOS a Fossil checkout therefore was not detected by tools/vcs, and the Fossil bridge could not resolve the repository from the checkout. Native Windows checkouts matched through _FOSSIL_ and are unchanged.
• The vcs test fixture created the same misspelled file, so the suite passed. It now creates .fslckout, and a new test resolves the repository through a .fslckout database.

Type of change

• Skill change (.claude/skills/<name>/) — eval fixtures updated below
• Tool / bridge contract (tools/<system>/*.md)
• Python package (tools/*/ with pyproject.toml)
• Groovy reference impl
• Cross-cutting (RFC, AGENTS.md, sandbox, privacy-LLM)
• Documentation (docs/, README.md, CONTRIBUTING.md)
• Project template (projects/_template/)
• CI / dev loop (prek, workflows, validators)
• Other: one-line marker-name fix in tools/fossil/README.md

Test plan

• prek run --all-files passes
• For Python packages touched: uv run pytest / ruff check / mypy passes (tools/fossil: 28 passed; tools/vcs: 21 passed)
• For Groovy bridges touched: command-line invocation tested end-to-end
• For skill changes: eval suite passes for the affected skill
• For skill behaviour changes: a new or updated eval fixture is included in this PR
• Other: both tests were run before the fix and failed (test_detect_marks_hg_svn_fossil with the corrected fixture, and the new test_find_repo_db_reads_fslckout), then pass with it.

RFC-AI-0004 compliance

No principle touched: the change renames a file marker and its test fixture.

Linked issues

None. Filed as a direct PR because the fix is a two-line rename plus tests.

Notes for reviewers (optional)

The tests use a hand-built .fslckout file and SQLite database rather than a real fossil open, as the existing fixture does. A test against the real binary (skipped when fossil is not installed, like the Mercurial tests) would check the marker against Fossil itself; I left it out to keep this PR to the rename.

[potiuk]

Correct, complete fix: every .fslckg probe in the tree now looks for Fossil's real POSIX checkout database .fslckout, and the corrected fixture plus the new find_repo_db test cover both code paths.

The new test resolves from a nested sub/ directory, so it pins the parent walk as well as the marker name. A follow-up test against a real fossil open (skipped when the binary is absent, like the Mercurial tests) would tie the marker to Fossil itself, as you noted — not needed for this PR.

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