feat(eve): linear - exclude other agents' threads from the dispatched turn

[lucasdoell]

Description

Closes #1226.

On multi-agent Linear issues, Linear's AgentSessionEvent.promptContext includes every agent's conversation threads (<other-thread> blocks), and linearChannel passed it verbatim as the turn message while unconditionally spreading previousComments into context — leaking other sessions' instructions and state into the model-visible turn.

This PR adds, per the design in research/linear-exclude-other-threads.md (included in this branch):

• linearChannel({ excludeOtherThreads: true }) — strips <other-thread> blocks from the default turn message. Fail-closed: on malformed or drifted markup (including a comment body embedding a literal </other-thread>), the message falls back to the session summary / issue title instead of leaking partial thread content, with a logged warning. An O(n) tag-count guard short-circuits adversarial inputs before the regex runs.
• onAgentSession can now return message (replaces the computed turn message; wins over the flag; empty/whitespace overrides are ignored) and previousComments (replaces the webhook's previous comments; [] drops them). The flag deliberately does not touch previousComments — those are the primary thread's own prior comments.
• New public exports stripLinearOtherThreads and LinearMessageOptions (options parameter on messageFromLinearAgentSessionEvent).
• Docs: new "Excluding other agents' threads" section in docs/channels/linear.mdx documenting the exposure and the composed-defaultOnAgentSession recipe.

How did you test your changes?

• pnpm --filter eve exec vitest run --config vitest.unit.config.ts src/public/channels/linear/ — 48 tests passing, including 21 new: fail-closed stripping (attribute-bearing fixtures modeled on Linear's documented format, unpaired-tag and embedded-literal attack cases), fallback chain, and full webhook→dispatch tests for the flag, override precedence, empty-override normalization, previousComments semantics, and image attachment on overridden messages
• pnpm test (full unit + integration suite), pnpm typecheck, pnpm lint, pnpm fmt, pnpm docs:check, pnpm guard:invariants

PR Checklist

• I linked an issue with prior discussion confirming this change is wanted
• I ran the relevant checks from CONTRIBUTING.md
• I added tests and documentation where relevant
• I added a changeset if this touches the published eve package
• DCO sign-off passes for every commit (git commit --signoff)

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