docs: complete HTTP brownfield PR-3 (agent guide, addendum, plan)

[HumanBean17]

Scope

Implements PR-3 from plans/PLAN-HTTP-ROUTE-METHOD-ENUM.md: agent-facing documentation and the v2 propose addendum for HTTP brownfield rename, CodebaseHttpMethod, inbound exclusivity (replace vs merge), verbose brownfield-exclusivity-shadowing INFO, and UC10 (silent Feign/framework disagreement).

Changes

• docs/AGENT-GUIDE.md: ontology calibration to 12; new Brownfield HTTP annotations (exclusivity) subsection; dual design-rationale links (MCP API v2 + HTTP-ROUTE-METHOD-ENUM-PROPOSE.md).
• propose/completed/BROWNFIELD-ANNOTATIONS-V2-ADDENDUM-HTTP-METHOD-ENUM.md: addendum linking parent v2 propose, HTTP propose, plan, and agent guide.
• plans/PLAN-HTTP-ROUTE-METHOD-ENUM.md: mark plan complete, PR-3 definition of done, tracking.

Behaviour / schema

• No Python, schema, or ontology code changes.
• Re-index: not introduced by this PR (PR-2 already documented).

Validation

• .venv/bin/ruff check . — passed.

Tests

• None (docs only); pytest not run for this PR.

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