feat(style): pounamu/ochre semantic palette for CLI output (#83)

[mattcollie]

Summary

Adds a semantic two-colour palette to Arai's CLI output (closes #83,
part of parent #64).

• src/style.rs — new module; single source of truth for all ANSI
  truecolor construction. Five public helpers: structural(), passage(),
  error(), warn(), should_colorize(). Palette: pounamu
  RGB(61,130,104) for structural text; ochre RGB(184,118,58) for
  passage moments, warnings, and errors. Foreground-only unconditionally —
  no background escapes anywhere.

• Gating hierarchy (highest priority wins): NO_COLOR present →
  strip all; non-TTY stdout and no CLICOLOR_FORCE → strip all;
  CLICOLOR_FORCE=1 → force on; interactive TTY → on. NO_COLOR
  dominates CLICOLOR_FORCE.

• Machine-consumer carve-out: the hook-protocol path
  (guardrails --match-stdin) and every --json flag output remain
  ANSI-free even when CLICOLOR_FORCE=1 is set. src/hooks.rs is not
  modified and does not import the style module — the carve-out is
  structural, not conditional.

• AC9 legibility decision: structural pounamu was lightened from
  the original RGB(31,77,63) (#1f4d3f) to RGB(61,130,104) (#3d8268)
  following a user-directed readability review. WCAG 2.1 contrast
  (verifier-computed): ~5.0:1 on black (passes WCAG AA >=4.5:1);
  ~4.2:1 on white (below WCAG AA-normal 4.5:1, above AA-large 3:1).
  The old value was ~2.3:1 on black (failing AA). The white-terminal
  gap (~4.2:1 vs 4.5:1) is a known caveat; AC9 calls for human visual
  inspection rather than a specific numeric threshold, and the new
  value is visibly improved on both common terminal backgrounds.

• Zero new dependencies: Cargo.toml is unchanged from origin/main.
  No colour crate, no terminal crate, no temp-dir crate.

Test coverage

Suite	Tests
Unit tests (src/)	400
brand_palette_verifier	30
style_integration	12
All other suites	94
Total	536 (all pass)

Full gate: cargo fmt --check clean, cargo clippy 0 new warnings,
cargo test 536/536.

Provenance

Built via the Taniwha dispatch cycle (design v5 → contract → leaf → verifier),
with a user-directed AC9 legibility correction (corrective leaf + verifier)
when the dispatcher's WCAG check flagged the original pounamu as too dark on
dark terminals. Full audit trail under .taniwha/kupu/ in this branch.

🤖 Generated with Claude Code

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