docs: codify the agent-ready issue standard

[Hmbown]

No-Issue: contributor-stewardship documentation standard.

Summary

• defines the executable issue structure for fresh agents
• documents verified-anchor discipline and the maintainer/community triage boundary
• links the standard from AGENTS.md and ISSUE_TRIAGE.md

Harvested from PR #4536 by @Hmbown. The unrelated keybindings follow-up from that draft is deliberately excluded.

Verification

• git diff --check origin/main...HEAD
• python3 scripts/check-coauthor-trailers.py --range origin/main..HEAD --check-authors
• link targets inspected with rg -n AGENT_READY_ISSUES AGENTS.md docs/ISSUE_TRIAGE.md

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[claude]

Claude finished @Hmbown's task in 48s —— View job

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Review: docs: codify the agent-ready issue standard

• Gather context (diff, changed files)
• Review AGENTS.md changes
• Review docs/AGENT_READY_ISSUES.md (new file)
• Review docs/ISSUE_TRIAGE.md changes
• Post final review

Summary: Docs-only change (no code, no Cargo.toml/CI touched). It defines the agent-ready issue standard, cross-links it from AGENTS.md and docs/ISSUE_TRIAGE.md, and — appropriately for a doc about anchor discipline — practices what it preaches.

Verified:

• .github/ISSUE_TEMPLATE/agent-task.yml (linked from the new doc) exists.
• Both relative links (AGENT_READY_ISSUES.md ↔ ISSUE_TRIAGE.md) resolve.
• The example anchor in "Required structure" (crates/tui/src/model_routing.rs::provider_router_candidates) is real — crates/tui/src/model_routing.rs:52.
• The example crate names (codewhale-tui, codewhale-config, codewhale-protocol) match the actual package names in their Cargo.tomls.
• git diff --check origin/main...HEAD is clean (no whitespace errors).

Notes (no changes requested):

• Since this is documentation with no code/provider-routing/test surface, items 1–5 of the standard review checklist (correctness, provider/model safety, reuse, tests, security) don't apply here.
• Nice touch that the doc's own worked examples are verified anchors rather than illustrative placeholders — consistent with the discipline it's documenting.

No issues found. This PR is ready to merge on its own merits.