Unify agent actions under Prompt Syntax

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Summary

• make standalone Prompt Syntax directives the only agent-authored mutation channel for Task Manager, project items, and pull requests
• preserve Codex message boundaries and fence state so directives cannot glue to prose or execute from quoted content
• replace Task Manager JSONL, checkbox mutations, items.inject, and prose PR URL harvesting
• publish the unified capability contract and bump the workspace to 0.1.56

Verification

• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all --check

Frontend vitest was unavailable in the clean temporary clone because dependencies were not installed; no installation was performed.

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Superseded by #76. The Prompt Syntax implementation landed there with review hardening, Codex live control, and the 0.1.56 bump isolated in its own commit. No unique changes remain here.