Fix root-path mentions minted from relative filenames in prose quotes

[TheGreatAxios]

Summary

• Path candidates inside non-path-shaped quoted prose must now open a new token, so "notes/plan.png" no longer mints a root-level /plan.png mention
• Unquoted scanning is unchanged

Verification

• bun run lint, bun run typecheck, and bun run build pass
• bun run test passes except sandbox-environmental failures (GPG, worktrees, sips) that reproduce on main; the new mention test passes

Fixes CL-7698

[linear-code]

CL-7698

[TheGreatAxios]

Review · Approve

Quoted prose no longer lets a mid-token / mint a root-level image mention; absolute paths inside quotes and unquoted scanning stay as before.

Findings

No findings.

Notes

• bun test ./src/tui/image-attachments.test.ts — 18 pass, 0 fail
• Commit subject/body style audit clean (git log origin/main..HEAD subject/body; length>72 filters empty)
• Scope: src/tui/image-attachments.ts, src/tui/image-attachments.test.ts only
• GitHub blocked --approve on this author's own PR; verdict here is still Approve