feat(skills,roadmap): 6 new bundled skills + parity roadmap additions

[emal-avala]

Summary

• Adds 6 new bundled skills: changelog, release, benchmark, coverage, migrate, docs
• Extends §7.7 with --bare hermetic mode and a defer permission decision for resumable headless pauses
• Extends §7.25 with managed-settings.d drop-in policy fragments and subprocess env scrubbing (AGENT_CODE_SUBPROCESS_ENV_SCRUB)
• Expands §7.28 lifecycle event set (file_changed, cwd_changed, instructions_loaded, post_compact, stop_failure, permission_denied, task_created), adds conditional if filters and subagent metadata in payloads
• Adds §7.30 MCP Elicitation, §7.31 Skill/Plugin Validator, §7.32 TUI Rendering and Transcript Search
• Small ergonomic bullets: agent initialPrompt frontmatter, effort frontmatter on skills/agents, Bash wildcard permission rule hardening

New skills

Each follows the existing bundled-skill style (short description + concrete instruction body).

Skill	Purpose
changelog	Stamp CHANGELOG.md from the current diff
release	Orchestrate version bump, CHANGELOG, tag per RELEASING.md
benchmark	Run benches and compare vs a baseline; flag >5% regressions
coverage	Run coverage tool, classify gaps, recommend 3-5 high-value tests
migrate	Analyze a dep upgrade or API migration; produce a call-site plan
docs	Sync rustdoc/README/mdx to public-API changes in the diff

Test plan

• `cargo check -p agent-code-lib --all-targets` clean
• `cargo test -p agent-code-lib --test skills_integration` — 6/6 pass (`>= 12` lower bound tolerates the new entries)
• `cargo clippy --all-targets -- -D warnings` (CI)
• `cargo fmt --all -- --check` (CI)