[Zhoujie628]

Closes #39

Summary

Round 1:

• chore: normalize license headers to the full Apache form - 54 files that had drifted to a short SPDX header; script-driven, no content change.
• docs: document public API surface and fix stale comments - javadoc for the workflow-definition model layer (contextFrom selection modes, step completion policies, route-candidate constraint, reserved terminal routes, snapshot isolation), six-endpoint controller list, package-info for all engine packages, @throws on validating contracts, LlmHelper demo fallback contract, neutral password placeholder in examples.
• docs: keep code comments English-only - removes the last non-English fragments from comments; string-literal business data is untouched.

Round 2 (fixes from verification review):

• chore: normalize remaining license headers - 14 files per branch carried a third variant (copyright + SPDX without the grant text).
• docs: fix comment accuracy findings - A2A-Version semantics corrected against the SDK validator (default 0.3, adapter-local alias), ExecutionResult.error no longer claims redaction, receivedMessages described as a structured projection rather than lossless, @throws tags moved off type-level javadoc into prose, root package javadoc attached to the declaration, runner package described as programmatic.

Verification

• Comments and javadoc only; no behavioral change.
• mvn javadoc:javadoc: zero warnings (four type-level @throws warnings in round 1 are gone).
• Full reactor mvn clean verify green on main and dev (including samples); 278 tests main, 351 dev.
• License header completeness validated against the grant-text URL line: main 186/186, dev 219/219.
• Comment scans (TODO markers, non-English text, internal-info leakage) clean.

CI impact

None: no production code changes; the only text change outside comments is the license header block.