feat(v0.6): namespace all 14 master slash commands under /master-<slug> prefix

Slash command discoverability cleanup. Claude Code users typically have
50+ skills installed; bare-word commands like /atisha and /zhiyi got
scattered across the /-completion list. v0.6 prefixes all 14 master
slash commands with master- so they cluster under /m<tab> and clearly
signal "this is a Master-skill master skill".

Scope of rename
---------------
* Slash commands: /zhiyi → /master-zhiyi, /huineng → /master-huineng,
  ... all 14 affected.
* Directory layout: prebuilt/<slug>/ → prebuilt/master-<slug>/ for all
  14 masters (git mv preserves history).
* Frontmatter: each prebuilt/master-<slug>/SKILL.md updates `name:` to
  master-<slug>.
* compare-masters and create-master meta-skills are intentionally
  unchanged — they're already prefixed by their nature, and
  /master-compare-masters would be doublespeak.

Decoupling: fojin.app/chat is NOT affected
------------------------------------------
The fojin web frontend's master dropdown uses bare slug IDs (atisha,
huineng, ...) and is already grouped under "法师模式" in its UI. Backend
master_profiles.py keeps `id="atisha"` etc. unchanged. No fojin-side
migration required. The two surfaces (Claude Code slash + fojin
dropdown) are now formally decoupled by design, not coincidence.

Compatibility
-------------
* NPX installer accepts both forms: `npx master-skill install zhiyi`
  (short) and `install master-zhiyi` (full) both resolve to the same
  prebuilt/master-zhiyi/ source. Install destination is always
  ~/.claude/skills/master-<slug>/. Backward-compatible uninstall
  handles legacy non-prefixed installs (~/.claude/skills/zhiyi/).
* The cli.mjs already used `master-${name}` for install destinations
  (since v0.3 NPX installer was added), so existing v0.4/v0.5 NPX
  users were already getting the prefix in skills/ — only the source
  prebuilt/ layout and slash commands change in v0.6.

Files updated
-------------
* 14 directories renamed (28 files moved, 0 content changes).
* 14 SKILL.md frontmatter `name:` fields.
* prebuilt/compare/SKILL.md: 43 slug references updated to prefixed form.
* bin/cli.mjs: resolveMasterDir helper accepts both short and full;
  cmdInstall and cmdUninstall handle legacy paths.
* .github/workflows/validate-and-test.yml: fidelity-smoke MASTERS
  rotation array updated to all 14 prefixed names (was 8 hardcoded
  汉传 only — now properly rotates across the full set).
* scripts/{validate,cite,query,test-fidelity}.py: --master arg help
  text examples.
* README.md + README_EN.md: situational guidance table, install
  snippets, master cards. New v0.6 release banner.
* SKILL.md (project-level) preset list with new slash names.
* ETHICS.md Tier table slug references (4).
* All plugin manifests bumped 0.5.0 → 0.6.0 with description noting
  the /master-<slug> invocation pattern.
* CHANGELOG.md: [0.6.0] section with breaking-change notice and
  migration commands for existing NPX users.

Validation
----------
* python scripts/validate.py --strict           ✓ 15 masters pass
* python scripts/validate-fidelity.py           ✓ all valid
* python scripts/test-fidelity.py --all --dry-run  ✓
* pytest tests/                                  ✓ 31 passed, 6 skipped
* node bin/cli.mjs list                          ✓ shows all 14 with
                                                   master- prefix
* node bin/cli.mjs install zhiyi                 ✓ resolves to
                                                   prebuilt/master-zhiyi/
* node bin/cli.mjs install master-zhiyi          ✓ resolves to same

Migration for existing v0.4/v0.5 users
--------------------------------------
    npx master-skill@0.5 uninstall zhiyi huineng xuanzang ...
    # OR: rm -rf ~/.claude/skills/{zhiyi,huineng,...}
    npx master-skill@latest install --all

Then start a new Claude Code session; the new slash commands are
/master-zhiyi, /master-huineng, etc.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

prebuilt/master-ouyi/sources/jiaoguan-gangzong-excerpts.md

@@ -0,0 +1,34 @@
+# 《教觀綱宗》关键片段
+
+> 蕅益大师著。CBETA ID: T46n1939。FoJin: https://fojin.app/texts/T46n1939
+> 本文件为教学引用用，节选自 CBETA 公开资料。完整经文请访问 FoJin 或 CBETA。
+
+## 化仪四教与化法四教
+
+原典（节选）：
+
+> 佛之设教，有化仪四教，化法四教。化仪如药方，化法如药味。化仪者，顿、渐、秘密、不定也。化法者，藏、通、别、圆也。
+
+**引用格式：**【《教觀綱宗》，T46n1939】→ https://fojin.app/texts/T46n1939
+
+**教义要点：**
+- 化仪四教：顿、渐、秘密、不定——说法方式
+- 化法四教：藏、通、别、圆——教义层次
+- 化仪如药方（怎么开），化法如药味（什么药）
+- 此为天台判教体系的纲要总论
+
+---
+
+## 圆教观法
+
+原典（节选）：
+
+> 圆教者，圆妙、圆融、圆足、圆顿。所谓"一色一香，无非中道"。观一念心即空即假即中，三观圆融，一心具足。
+
+**引用格式：**【《教觀綱宗》，T46n1939】→ https://fojin.app/texts/T46n1939
+
+**教义要点：**
+- 圆教为天台判教之最高
+- 一色一香无非中道
+- 一心三观圆融，非次第
+- 蕅益大师以此天台圆教为教理框架