Define bot implementation workflow and document the issue-to-PR lifecycle

[d3cker]

The bot instructions previously described execution boundaries but did not define how implementation sessions should plan, delegate, verify, and review changes. This PR adds that process while preserving tool-free triage, structured outputs, GitHub question and permission handling, and dispatcher ownership of publication.

• Scope instructions to triage, branch selection, implementation, native workers, media helpers, and PR-title generation. Guide nontrivial implementation through planning, available workflows and subagents, verification, and final review.
• Document the implemented issue-to-PR lifecycle with eight Mermaid diagrams, including questions, follow-up rounds, checkpoints, retries, publication, merging, and tab closure.
• Expand AGENTS.md with a documentation guide and source map, and update runtime guidance. Planning and review remain model instructions within the existing running phase, not new dispatcher phases or enforced publication gates.

Validation: node --import tsx --test test/prompt.test.ts (3 passed); git diff --check; documentation link checks. The section 4 sequence diagram passes VS Code's bundled Mermaid parser. No live GitHub/OpenCode bot execution was performed, so model adherence to the expanded instructions is not yet integration-tested.