docs: add OIDC integration guide

[basauth]

Covers required environment variables (OIDC_ISSUER, OIDC_AUDIENCE), optional OIDC_JWKS_URI override, JWKS discovery behavior, supported algorithms (RS256/ES256 family), rejected algorithms (alg=none, symmetric), startup/readiness behavior, local-dev note, and subject identity warning.

Summary

This PR adds the OIDC Integration Guide as a reference

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Checklist

Step 1 — Identify affected surfaces

Check every surface this PR touches:

• Authentication or token verification logic (src/basis_gateway/auth/)
• Identity normalization or subject mapping (src/basis_gateway/auth/subject_mapper.py)
• Audit event emission or gateway event vocabulary (src/basis_gateway/audit/)
• Readiness component registration or behavior (src/basis_gateway/readiness.py)
• API request/response schemas (src/basis_gateway/api/schemas.py)
• Environment variable configuration (src/basis_gateway/config.py)
• Correlation ID handling (src/basis_gateway/middleware/)
• Public documentation (README.md, docs/)
• None of the above — this PR does not touch any of these surfaces

Step 2 — Classify the change

• Additive only (new behavior, new config option, new audit event — existing callers unaffected)
• Breaking (changes API contract, removes config support, alters audit event shape, changes authentication behavior)
• Not applicable (docs, tests, refactor only)

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If the change affects authentication or audit behavior

• New or changed behavior is covered by tests
• Audit event emission for affected paths verified (all pre-evaluation failure paths must emit a gateway event)
• Security model assumptions in SECURITY.md and docs/release-readiness.md still hold

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If the change modifies basis-core integration

• Change uses only the stable public API (basis-core/docs/public-api.md)
• No direct imports from basis_core internals

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Tests

Describe what tests cover this change, or explain why no new tests are needed.

• pytest passes locally
• ruff check passes
• ruff format --check passes
• mypy src passes