feat(glossary): add the glossary utility skill + extractor agent

Gives the plugin an executable procedure over a project's glossary,
closing the two gaps the glossary-convention infrastructure left open:
no on-ramp to build a glossary for an existing project, and no
conformance check around hand-maintenance.

New `glossary` utility skill (invoked on demand like `issue`, not a
pipeline phase) with a mode dispatch:
- maintain: guided add / change / remove of one entry under a local
  conformance check (three-field shape, <=2-sentence definition,
  collision scan against existing entries) plus a stale-usage sweep on
  change/rename. A non-conforming entry is rejected with the named rule,
  not written. The sweep reports drift; it does not block (guidance, not
  enforcement).
- bootstrap (user-only): fans out one read-only glossary-extractor agent
  per prose slice, merges and clusters their observed variants, auto-
  includes clear winners, surfaces contested clusters to the user for the
  canonical choice, and leaves unresolved clusters out (never coined).

New `glossary-extractor` agent (read-only, template-conforming): sweeps
one prose slice, reports recurring concept terms + competing variants
with frequencies and locations, coins nothing.

Single-sourcing held throughout: the skill applies the rules in
docs/glossary-convention.md and restates none of them. Authority is
unchanged — maintain keeps the user-any-time / boss-record-reality rule;
bootstrap is user-only, with boss allowed to recommend it via bounce-back
(one wired sentence in boss/SKILL.md). docs/glossary-convention.md gains
a one-line tooling pointer to the skill; README.md lists `glossary` and
`issue` as utility skills and drops the stale 'Eight skills' count the
fresh-context grounding-check flagged (it had silently omitted `issue`).

install.sh links the new skill + agent with no installer change (its glob
covers any top-level dir with a SKILL.md + agents/ subdir; verified).

Docs/prose-only repo, no test runner: all 13 grep presence-gates green
against verified zero baselines; final sweep ALL_PRESENT, install.sh
still parses (bash -n). Implements
docs/specs/2026-05-31-glossary-skill-design.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/glossary-convention.md

@@ -64,3 +64,7 @@ reality, never to invent**:
 The orchestrator never coins a brand-new term into the glossary: a
 term earns its entry by already being in consistent use. All other
 skills and agents are read-only consumers of the glossary.
+
+The `glossary` skill (`../glossary/SKILL.md`) is the executable procedure
+for building and extending a glossary under these rules: this file stays
+the rules, the skill applies them.