refactor: consolidate duplicated backend helpers into internal/session

[sorafujitani]

Context

With four backends (claude/opencode/grok/codex) the same small pure helpers have been copy-pasted across packages. Any behavior tweak (e.g. the "future timestamps sink to the bottom" rule) now has to be applied in up to 5 places.

Current state (verified locations)

• sortEpoch — 5 copies:
  • internal/source/all.go:245
  • internal/codex/codex.go:473
  • internal/grok/grok.go:494
  • internal/opencode/scan.go:131
  • internal/session/scan.go:145 (this one takes *Session instead of int64; same rule)
• pathIsDir — 4 copies: internal/codex/codex.go:492, internal/grok/grok.go:532, internal/opencode/scan.go:138, internal/session/parse.go:278
• allowed — 3 copies: internal/codex/codex.go:480, internal/grok/grok.go:520, internal/opencode/scan.go:103
• idCorruptsRow — 3 copies: internal/codex/codex.go:488, internal/grok/grok.go:528, internal/opencode/scan.go:99
• firstNonEmpty — 2 copies: internal/grok/grok.go:511, internal/session/parse.go:220
• The sort.Slice block using sortEpoch + "ID ascending tiebreak" is also duplicated next to each sortEpoch copy (e.g. internal/session/scan.go:131-138).

Task

1. Add a new file internal/session/helpers.go with exported versions:
   • func SortEpoch(epoch, nowEpoch int64) int64
   • func SortByRecency(out []*Session, nowEpoch int64) — encapsulates the full sort.Slice with the ID tiebreak
   • func PathIsDir(path string) bool
   • func Allowed(allow map[string]struct{}, id string) bool
   • func IDCorruptsRow(id string) bool
   • func FirstNonEmpty(values ...string) string
     Copy the bodies from the existing implementations verbatim (they are identical); keep the existing doc comments, especially the future-timestamp rationale on sortEpoch.
2. Replace every listed copy with a call to the shared function and delete the local definitions. All these packages already import internal/session, so no import cycle is possible.
3. Replace each duplicated sort.Slice block with session.SortByRecency(...).
4. Add a table-driven test file internal/session/helpers_test.go covering each helper (include: future epoch clamps to 0, nil allow map, empty values for FirstNonEmpty, tab/newline cases for IDCorruptsRow). Move/merge any existing per-package tests of these helpers here and delete the duplicates.

Acceptance criteria

• grep -rn "func sortEpoch\|func pathIsDir\|func allowed(\|func idCorruptsRow\|func firstNonEmpty" internal/ --include="*.go" returns only the new shared definitions (plus tests).
• No behavior change: go test ./..., go vet ./..., golangci-lint run, gofmt -l . all pass/clean.