feat: update to mcpp 2026.8.1.1, ship templates/, i18n READMEs, split CI per platform - #4

What

Brings the template repository up to the current mcpp release and gives it the three things a
library scaffold was missing: shippable project templates, multilingual docs, and per-platform CI.

1. Latest mcpp (0.0.28 → 2026.8.1.1)

.xlings.json pins the current release. Everything was re-verified against it locally —
mcpp build, mcpp test, examples/basic — and the docs were re-synced with what that
version actually does: dev is the default build profile now, lib targets are inferred from
src/*.cppm, [lib] path overrides the lib-root convention, mcpp build ignores
[dev-dependencies], and the C++ standard belongs in [package] standard.

2. templates/ support

The library now ships project templates, so downstream users can scaffold from it:

mcpp new --list-templates mylib
mcpp new myapp --template mylib        # basic (default)
mcpp new mylib2 --template mylib:lib

tools/template_smoke.sh renders each template the way mcpp new does, repoints the
dependency at the checkout, then builds it (and runs it when it produced a binary) — so
templates are verified before a release exists in the package index. CI runs it on all
three platforms.

3. README in English (default) / 简体中文 / 繁體中文

README.md, README.zh.md, README.zh.hant.md with a language switcher; docs/architecture.md
gets the same three. Also added:

• a note that xlings install installs mcpp into the project environment (pinned by
  .xlings.json), and xlings install mcpp -g is the global form;
• a publishing section: mcpp publish --dry-run → tag → release asset → mcpp-index PR,
  with links to the index repo, its contributor docs and its add-mcpp-index-package skill;
• reference projects: mcpplibs/llmapi and
  mcpplibs/cmdline, each with a line on what to read it for.

4. CI split per platform

ci.yml → ci-linux.yml / ci-macos.yml / ci-windows.yml. Each runs the same path a new
user walks: install → build → test → example → template smoke. One badge and one log
per OS, so a macOS or Windows regression cannot hide behind a green Linux run.

5. Agent skills

New .agents/skills/mcpp/SKILL.md (commands, mcpp.toml fields, conventions, shipping
templates/) and .agents/skills/mcpp-index/SKILL.md (finding dependencies, namespace
resolution rules, publishing). more-details and the skills README were updated for the new
layout.

Verification

Run locally against mcpp 2026.8.1.1 (Linux x86_64):

• mcpp build — lib target inferred, ok
• mcpp test — 1 passed, 0 failed
• cd examples/basic && mcpp run — prints hello mcpplibs
• bash tools/template_smoke.sh — both templates build; basic runs; the generated lib
  project also passes its own mcpp test
• all relative links in the READMEs/docs/skills resolve; external links return 200
  (the CI badges 404 until this lands on main, as expected)

macOS and Windows are validated by this PR's own CI runs.