feat: add partials for shared, inlined text fragments across skills

[steve-calvert-glean]

Summary

Skills that need the same procedural prose repeated across many files (a consistent auth flow — "try OAuth first, fall back to a token, or ask for the user's email and look up credentials" — is the motivating case) had no reuse mechanism. This adds a partials/ directory of reusable {{> name}} text fragments, resolved once at pluginpack build time.

• source.partials config option: a project-level directory (not scoped to one source plugin), mirroring source.skills's shape.
• Real mustache library (new production dependency, @types/mustache in dev), not a hand-rolled parser — gets missing-partial-renders-empty and clean standalone-line whitespace handling for free, both verified directly against the Mustache spec's own test suite before deciding.
• View is always {} — no config/env data is ever exposed to interpolation. An earlier, broader "dynamic value injection" idea (secrets, env-specific config) was explicitly scoped out: build output is committed to a repo the target host installs from, and baking secret-like values into that committed output would be a real security footgun. This is pure text reuse, full stop.
• Hooks into collectPluginFiles (render.ts) and withRootFiles (targets/engine.ts) — covers skills/agents/commands/rules, additionalFiles, and a target's rootFiles.
• Partials may reference other partials (native recursive composition); a circular reference is the one thing pluginpack itself catches, since Mustache.js has no protection against it — via dependency-graph cycle detection at load time, before any render call.
• Known, documented limitation: because substitution is real Mustache rendering, other {{...}}-looking text in the same file (e.g. a skill documenting Handlebars/Jinja/Mustache itself) is also processed against the empty view and typically disappears. Considered and rejected a narrower hand-rolled {{> }}-only parser to avoid this — reproducing Mustache's standalone-line whitespace handling and nested-partial resolution by hand means reinventing battle-tested infra a mature library already solves correctly.

Test plan

• npm run typecheck
• npm run lint
• npm test (92/92 — 12 new tests: happy path, missing partial, nested composition, circular reference, multi-marker, cross-plugin sharing, no-op backward compat, byte-safety, duplicate name, missing directory, rootFiles, documented collision behavior)
• npm run audit (production-scoped) — 0 vulnerabilities, mustache itself is clean
• npm run build && node dist/cli.js docs --check
• npm run docs (includes markdown-code check)
• npm pack --dry-run
• Manual smoke test: built a fixture with source.partials configured against claude + cursor targets, confirmed the substituted auth-flow text reads naturally in context in both, and pluginpack validate passes for both