docs(agents-md): shrink AGENTS.md and prefer plugin skills over shell

AGENTS.md had grown to duplicate content owned elsewhere: behavioral
rules already active globally via ~/.agents/AGENTS.md, a VISION.md
read-on-demand entry CONTEXT.md already covers at session start, and
setup/testing/commit instructions that explained hook mechanics the
git plugin's pc-run/git-commits skills already own. It also gave no
explicit steer toward using installed plugin skills over raw shell
commands, so agents defaulted to shelling out to git directly.

- Added a "Prefer plugin skills over raw shell" section mapping
  operations (commits, branches, hooks, issues/PRs, linting, AGENTS.md
  itself) to the skill that owns them.
- Collapsed Setup/Testing/Commit-conventions into one section, keeping
  only the two genuinely non-obvious gotchas (missing
  default_install_hook_types, bats submodule auto-init).
- Removed the "Subagent orchestration" section: its content was mostly
  universal Agent/Task/worktree-tool facts, not specific to working in
  this repo, so it moves to core/instructions/subagent-orchestration.md
  (deployed globally via install.sh, referenced from core/AGENTS.md's
  content index) rather than staying repo-local.
- Removed agentsmd-author's "not this repo's own" scope exclusion in
  CONTEXT.md (ADR-0012 never mandated it) so this task could route
  through it, and folded the forge-routing rule it left behind into
  CONTEXT.md's existing Skill composition entry.

AGENTS.md: 50 -> 40 lines. Full test suite and manifest check pass.

core/AGENTS.md

@@ -23,3 +23,4 @@ Read these files on demand:
 
 - **Coding conventions** (`~/.claude/core/instructions/coding.md`) — when writing, editing, or reviewing code
 - **Testing conventions** (`~/.claude/core/instructions/testing.md`) — when writing or running tests
+- **Subagent orchestration** (`~/.claude/core/instructions/subagent-orchestration.md`) — when spawning or coordinating subagents/forks