feat: copy Forge-aware issue handoffs

[Codename-11]

Summary

• add a compact Copy handoff action to issue detail headers
• copy the issue key, quoted reference title, and canonical human-readable Forge URL
• include concise Forge MCP and Delivery instructions for a receiving local agent
• cover the prompt contract with Vitest and the real clipboard flow with Playwright

Behavior

The copied handoff tells the receiving agent to inspect current Delivery ownership, open or continue the appropriate run, and—for code—continue or explicitly claim/join/handoff one isolated issue session instead of creating competing work. It also preserves native IMPLEMENTS PR linkage, visible status/heartbeat/completion behavior, and explicit operator authority for merge/release/deploy.

Issue content is labeled as reference data rather than operational instruction. Copying does not mutate assignment, runs, or Delivery ownership.

Validation

• pnpm vitest run tests/unit/issue-agent-handoff.test.ts
• pnpm lint (passes with pre-existing warnings)
• pnpm typecheck
• pnpm docs:build
• E2E_FORCE_FRESH_SERVER=1 pnpm exec playwright test tests/e2e/issue-flow.spec.ts --workers=1
• git diff --check

Delivery

Implements AXI-156. No version bump, merge, release, or deployment is included.