fix: support Tia when the Pest project is a subdirectory of the git repository

[johnbacon]

What:

• Bug Fix
• New Feature

Description:

Tia currently cannot be used when the Pest project is a subdirectory of its git repository (e.g. a backend/ Laravel app in a monorepo). On 4.x this failed silently — change detection mapped every change to zero tests and replayed stale results as passing (#1783's reproduction) — and 5.x added the TiaRequiresRepositoryRoot guard, which refuses to run instead (#1802).

The root cause is a path-frame mismatch: git prints and addresses paths relative to the repository root, while the Tia dependency graph is keyed on project-relative paths. This PR makes ChangedFiles translate at every git boundary instead of requiring the two roots to coincide:

• ChangedFiles::gitPrefix() resolves the project's prefix inside the repository once, via git rev-parse --show-prefix (empty at the repo root or when git is unavailable, so single-repo projects are byte-for-byte unaffected).
• Output paths from git diff --name-only and git status --porcelain are stripped to project-relative; paths outside the project (sibling packages) are dropped.
• Input paths to git show <sha>:<path> (behaviourally-unchanged filtering, and the composer.lock drift summary in the Tia plugin) get the prefix prepended.
• git check-ignore --stdin already receives cwd-relative paths with the project root as cwd, so it needed no change.
• The TiaRequiresRepositoryRoot guard and exception are removed — with the translation in place there is nothing left to guard against. The suggested-fix sketch in TIA: change detection misses all changes when the Pest project is a subdirectory of the git repository (monorepo) #1783 is what's implemented here.

Covered by new ChangedFiles unit tests that build real nested-repo fixtures: prefix resolution, working-tree + committed change detection from a subdirectory (sibling changes dropped), the behaviourally-unchanged filter across the prefix boundary, and a repo-root regression case. Full suite (--exclude-group=integration), Pint, Rector, and PHPStan are green locally.

One adjacent gap deliberately not touched here: BaselineSync reads <projectRoot>/.git/config directly to derive the GitHub slug, so remote baseline sync stays inactive for subdirectory projects (and git worktrees, where .git is a file). Switching that to git config --get remote.origin.url would fix both — happy to follow up separately if you want it.

Related:

• Fixes TIA: change detection misses all changes when the Pest project is a subdirectory of the git repository (monorepo) #1783
• Closes the scenario in [Bug]: --tia unusable in monorepos #1802 (closed as duplicate)

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