fix(planning): reject non-canonical gateway signatures

[seonghobae]

RCA

A current exact-head CI failure on the Planning workspace-authority regression exposed an encoding-alias edge case. A SHA-256 HMAC encoded as 43-character base64url has unused low bits in its final character. Node's base64url decoder accepts non-canonical textual aliases whose decoded bytes are identical to the canonical signature. The controller-level tamper regression therefore had a low-probability path where changing only the final character did not change the decoded digest, and the service accepted the altered header.

This is both a flaky-test root cause and a fail-closed authority-integrity gap: signature text should have one canonical representation.

Test-first repair

1. Add a deterministic regression that constructs a non-canonical final-character alias and proves it decodes to the same bytes as the canonical HMAC.
2. Require requireTrustedWorkspaceContext to round-trip the decoded digest back to the exact original base64url text before constant-time HMAC comparison.
3. Preserve the existing length/character, TTL/skew, UUIDv4, secret-availability, and constant-time digest checks.

Scope

Two Planning HTTP-boundary files only. No workflow, dependency, migration, persistence, credential, reviewer, or branch-policy changes. This does not overlap active Planning data-rights PR #179 files.

Acceptance

Require exact-head Planning/full CI, merge-tree compatibility against independently resolved live main, AppGuardrail, SAST Semgrep, Security Scan, Commercial Readiness, CodeRabbit/current review findings, and unchanged-head revalidation before merge.

Refs #176 (observed blocker evidence).

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📥 Commits

Reviewing files that changed from the base of the PR and between 65373a7 and 672b7a1.

📒 Files selected for processing (2)

• apps/planning-service/src/http-boundary.test.ts
• apps/planning-service/src/http-boundary.ts

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