refactor(hooks/builtins): inline & simplify add_prompt_files

[dgageot]

Moves session.ReadPromptFiles (and its private helpers) into the only
package that consumed it — pkg/hooks/builtins/add_prompt_files.go —
then simplifies the architecture and tightens up the tests. No
production behavior change.

Commits

1. refactor(hooks/builtins): inline readPromptFiles into add_prompt_files

   • Folds session.ReadPromptFiles, findFileInHierarchy, and
     isFile into the builtin as unexported helpers; drops the
     pkg/session import.
   • Relocates the seven prompt-file tests as internal tests in
     pkg/hooks/builtins.
   • Deletes pkg/session/prompt_file{,_test}.go.

2. refactor(hooks/builtins): split discovery from I/O in add_prompt_files

   • Replaces the awkward middle layer readPromptFiles(workDir, filename) ([]string, error)
     (which mixed path discovery with I/O just to bubble a swallowed
     error up to the hook) with promptFilePaths(workDir, filename) []string
     — pure path discovery, no error return.
   • Hook now owns all I/O and the log-and-skip policy in one place:
     a single os.ReadFile + slog.Warn site.
   • Dedup uses an explicit slices.Contains instead of an inline path
     comparison.
   • Tests retargeted at promptFilePaths, asserting on discovered
     paths (more precise than round-tripping byte content). Content
     reading remains covered end-to-end by TestAddPromptFiles* in
     builtins_test.go.

3. test(hooks/builtins): make prompt-file tests hermetic via injected homeDir

   • Fixes a real test-quality issue carried over from the original
     pkg/session tests: a homeFixture helper that wrote into the
     real \$HOME and could silently overwrite a user file with a
     colliding name (skip-cleanup-when-it-existed semantics).
   • promptFilePaths(workDir, homeDir, filename) now takes
     homeDir explicitly; the hook resolves it once via
     os.UserHomeDir() (errors → "", which disables the home
     branch). Same observable behavior in production.
   • Tests pass t.TempDir() for both `workDir` and `homeDir`,
     so they never touch the real user environment. Drops the
     homeFixture helper, the t.Skip, the unique-filename hack,
     and the conditional cleanup.
   • Adds two tests the previous shape couldn't express:
     TestPromptFilePathsWorkDirNestedInHome (workdir as a
     subdirectory of home, dedup via the hierarchy walk) and
     TestPromptFilePathsEmptyHomeDirSkipsHomeLookup (explicit
     disable contract).

Validation

• `golangci-lint run ./...` → 0 issues
• `go run ./lint .` → 777 files inspected, no offenses
• `go mod tidy --diff` → clean
• `go test ./...` → all packages pass