feat(skills): per-skill runtime compatibility (#119)

[HongmingWang-Rabbit]

Summary

• SKILL.md frontmatter can now declare `runtime: [claude-code]` or `runtime: [hermes, claude-code]` to opt out of incompatible adapters at load time instead of failing at first invocation
• Default `["*"]` means universal — existing skill libraries need zero migration
• Borrowed from hermes' declarative skill-compat pattern (one of three patterns surfaced in the hermes architecture survey under fix(security+scheduler): IPv6 SSRF gap + scheduler unit tests [supersedes #111, #112] #119; event-log + observability config block stay open)

Wiring

• `SkillMetadata.runtime: list[str] = ["*"]` — new dataclass field
• `_normalize_runtime_field()` — accepts list, string sugar, missing → `["*"]`; malformed warns + falls back to universal so a typo never silently drops a skill
• `load_skills(..., current_runtime=...)` — filters out skills whose runtime list lacks `"*"` or `current_runtime`, with INFO log line
• `BaseAdapter.start()` passes `type(self).name()` so the live adapter drives the filter
• `SkillsWatcher` takes the same kwarg so hot-reload honors it

Test plan

• 32 skill-loader tests pass (24 existing + 8 new)
• New tests cover: default universal, no-field universal, explicit match, explicit mismatch (skip), string sugar, wildcard short-circuit, `current_runtime=None` preserves old behavior, malformed-warns-not-drops
• CI green
• Merge after CI; the remaining two fix(security+scheduler): IPv6 SSRF gap + scheduler unit tests [supersedes #111, #112] #119 patterns will land in separate PRs

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