docs: add acceptance matrix as a hard rule across roadmap, AGENT, and README

[jamiesun]

Summary

Roadmap documents must now include an acceptance matrix. This PR adds a Capability Coverage Matrix as the mandatory acceptance section of the roadmap, codifies its coverage floor as MUST-level hard rules in AGENT.md, and links both from a key position in the README — bilingual throughout.

docs/ROADMAP.md / docs/ROADMAP.zh.md

• New Acceptance Matrix (Capability Coverage Matrix) section as the roadmap's acceptance chapter (single source of truth).
• Five coverage-floor rules written as hard rules:
  1. Every top-level capability has at least one happy-path E2E.
  2. Every high-risk capability covers at least one failure path.
  3. Every permission-gated capability is verified under at least two roles (mapped to policy modes: readonly + one of guarded/unrestricted).
  4. Every state-changing operation verifies recovery/rollback after at least one failure.
  5. Every new top-level capability ships with its E2E and a matrix row.
• 14 capability rows with risk level and evidence citing real test names (src/*.zig) and scripts (playground/scripts/, scripts/).
• Honest gaps recorded instead of invented coverage: ❌ daemon happy-path E2E, ❌ serve run method round-trip, plus one "to verify" (schedule failure recovery) — each with a minimum expectation.

AGENT.md / docs/AGENT.zh.md

• New Acceptance Matrix (Hard Rules) section: the five floors as MUST-level requirements, linking to the matrix (rules + link only, no matrix duplication).
• Extension workflow gains step 4: adding/removing a top-level capability must update the E2E and the matrix, otherwise the change is incomplete.

README.md / docs/README.zh.md

• New Quality & acceptance section linking the matrix and the hard rules, with a one-line statement of the coverage floor.

Notes

• The matrix constrains what must be proven, not which framework or test layout to use.
• Docs-only change; verified table column consistency, GitHub anchor slugs, and that every cited test/script exists.
• Local CI skipped on push per AGENT.md exception for docs-only changes (no mdBook content touched).