docs(agents): record how to retire a published endpoint past the contract gate

[ExtraToast]

Summary

Records the mechanic that let #79 go from 17 contract findings to zero, so the next person retiring a published endpoint does not have to rediscover it.

Nothing in the repo said how to remove an endpoint that oasdiff guards. The gate has no waiver, so the answer is procedural: deprecate in one PR, remove in the next, because the base only gains the deprecated: true marker once the first lands. A path absent from the base is then free to introduce in any shape.

Also records two smaller traps: a required header added to an existing endpoint is breaking even when the endpoint needs auth it never had (declare it optional and refuse on absence), and oasdiff's real behaviour is worth checking against the binary with two fixtures rather than reasoning from its documentation.

Validation

:api:ktlintCheck clean. Documentation only — no code, no spec, no workflow change.

Notes

• Written from what actually happened across chore(agents): deprecate the legacy Conversation endpoints #81, feat(agents): drop the legacy Conversation/Message model #82 and refactor(agents): rename Chat Session to Conversation, drop the legacy model #79 rather than from the tool's docs. Each claim was established by running oasdiff against purpose-built fixtures: removing a deprecated path reports nothing, removing an undeprecated one reports api-path-removed-without-deprecation, and a brand-new path carrying a required header, a nullable property and a narrower maxLength reports nothing.
• The note about verifying against the binary is there because a first pass at this problem reasoned from documentation and concluded no route existed. The route was found by testing the tool.

refs #68