v0.7.0 — agent interface — JSON envelope, plan/apply --plan, error taxonomy

Features

• Typed error taxonomy (StitchError / FailureClass): 14 stable exit
  codes (0–13) with class names, messages, and resolution hints. Text mode
  prints hint: lines; --json envelopes include class, code, message,
  hint, and details.
• JSON envelope with a stable schema: 1. Global --json flag on
  status, list, doctor, prune, render, apply, diff, and plan.
  The envelope (including error) goes to stdout; the exit code stays honest;
  stderr is reserved for hook/subprocess output.
• stitch render <store>/<file>: read-only in-memory template render to
  stdout; --json emits {source, link_name, sha256, content}.
• stitch plan: capture apply's exact op list as a stitch/plan JSON
  file. Includes staged-render hashes, config hash, and platform fingerprint.
  Text mode emits the raw plan; --json wraps it in the envelope.
• stitch apply --plan <file>: verbatim plan execution with preflight +
  per-op precondition re-checks + stale-plan detection. Aborts at the first
  failed op. --dry-run runs validation with no mutations. --plan is
  incompatible with --only and --force (usage error, exit 2).

Trust & safety

• Plan files are untrusted input. apply --plan validates every op
  against the pinned config: path traversal rejection, backup path validation
  (same directory as target), source under repo, target under configured
  target, and .stitch/render/ gitignore guard.
• No plaintext in plans. stage_render ops pin only the SHA-256 of the
  approved render; rendered content is not stored in the plan.
• Stale-plan refusal. Config edits, state.toml edits, platform drift, or
  render-hash drift cause apply --plan to exit 12 before any mutation.
• Honest exit codes for agents. diff/plan exit with the conflict/error
  class; apply aggregates per-entry classes (single → that code, multiple →
  11); doctor exits 13 on error-level findings.

Internals

• Refactored compute_plan. apply, diff, plan, and the JSON/text
  renderers all consume the same Plan struct, so the two views cannot drift.
• New dependencies: serde_json, sha2.