Modernize GitHub Actions workflows: Node 24 majors + pin floating versions (#90) - #97

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* Restructure agent guide: split by audience, drop closed milestones

AGENTS.md had accumulated milestone archaeology and duplicated most of
docs/contributing/development.md, so the two drifted apart — development.md
held the more current dependency table and project structure.

Split them by audience. AGENTS.md keeps conventions, architecture, and
design decisions; development.md keeps contributor setup (build/test
matrix, directory tree, dependencies). Each cross-links the other rather
than restating it, so every fact has one owner.

Dropped from AGENTS.md: M0-M7 milestone history and the M6/M7 narratives
(CHANGELOG owns release history), the full build/test matrix, the
directory tree, the gh CLI tutorial, and five roadmap items that never had
tracking issues (MCP server mode, community catalog, remote catalog
search, WASI/Docker backends, LRU usage tracking).

Corrected stale claims:
- santhosh-tekuri/jsonschema was listed as a planned dependency but was
  never adopted; validation is bespoke in internal/skill/validator.go
- per-platform project capacity was documented as counting
  <platform>/.skern/skills/ rather than the platform's own project dir
- "Milestones M0-M7 are complete (v0.3.0)" contradicted the v0.3.1 header
- the feature/m<N>-<slug> branch convention has been unused since #81;
  actual practice is <type>/<slug>

Added a Scope section recording where the project stands: the registry is
the center of gravity, while platform install/uninstall stays first-class
for native progressive disclosure, sandboxed agents, and migration off an
existing .claude/skills/. The cobra group split in internal/cli/skill.go
already encodes this. Also documented tag validation and the
--tag/--category filter surfaces, which were undocumented, and noted the
#100 frontmatter gap (installs are unaffected — they are byte copies).

Amended design decision 3: adapters remain declarative, but pure-copy is
no longer an invariant. A Spec may carry post-install behavior where a
platform's loader requires it, with Uninstall reversing whatever Install
produced. #99, #101, and #47 all need this mechanism; settling it once
avoids three special cases. Stated as direction only — the adapters still
copy today, and docs/concepts/platform-adapters.md still describes that.

Roadmap now lists only tracked issues, grouped by theme.

Also: skern skill --help advertised 3 of 8 platforms from a hardcoded
string; it now derives the list from the spec table so it cannot drift
again. CHANGELOG [Unreleased] was empty despite #98, #95, and #90/#97
having shipped.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* Record deferred-scope decisions, fix remaining roadmap drift

- AGENTS.md Scope: note that remote discovery/catalog was considered and
  closed (#19, #50) in favor of skild/skills.sh, with #79 covering import;
  flag MCP mode, execution backends, and usage eviction as untracked.
- docs/guide/index.md: stop advertising MCP mode and remote catalog search
  as roadmap items; point at AGENTS.md for scope.
- development.md: branching/commit conventions now live only in AGENTS.md
  (Git Workflow); drop the duplicate bullets and redundant trailer.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 5 <noreply@anthropic.com>