fix: warn on invalid auto-approve-risk header; require config in write tools

[thisrohangupta]

Summary

Two hardening fixes for the write-tool security path (harvested from the still-valid #207, re-done cleanly on current main):

• session-headers.ts — log a warn when X-Harness-Auto-Approve-Risk holds an unrecognized value (e.g. read, a typo) instead of silently falling back to the deployment default. A misconfigured client otherwise ends up with a different security posture than it intended, with zero signal. The offending value is surfaced in the log to aid debugging.
• Write-tool registrars (harness_create, harness_update, harness_delete, harness_execute) — change config?: Config → config: Config. registerAllTools always passes it (src/tools/index.ts), so optional was a type lie: a future caller could omit it and silently bypass HARNESS_READ_ONLY enforcement and the session auto-approve setting. The now-needless ?. guards at the read-only / auto-approve sites are simplified.

Context

The changes were originally proposed in #207 (a follow-up to #204 whose two review nits never merged). That PR is conflicting, CI-failing, and CLA-unblocked, so this re-implements the same intent on a fresh branch off main and fixes the stale test it flagged. #207 can be closed as superseded.

Test plan

• pnpm build, pnpm typecheck
• pnpm docs:generate && pnpm docs:check (README up to date)
• Added session-headers.test.ts coverage: warns on an unrecognized value (surfacing it), does NOT warn on a valid value or when the header is absent
• Updated all write-tool registrar call sites across the test suite to pass config (the previously-optional param)
• Full suite green: 118 files, 2513 tests, 0 failures

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