docs: teach the theme skill formatVersion and themes that can't load

[omartelo]

Follow-up to lichdotdev/lich#598 and lichdotdev/lich#600. Depends on both shipping in a release: until #598 a lich in the wild still logs broken themes instead of listing them, and until #600 it ignores minLichVersion and a manifest formatVersion.

What changes

• skills/theme/SKILL.md: the rules list gains formatVersion (optional, omitted means 1, positive integer no greater than the format lich reads, refused by number). The Traps bullet about copying a file into the themes directory now says a broken stored theme is listed under the theme strip as one that can't load, with the reason and Remove, after a window reload; only a file whose name is not a valid theme id stays log-only. Import is still the path that selects the theme. The frontmatter description drops "silently missing" in favor of "listed as a theme that can't load, or missing".
• skills/theme/validate.mjs: accepts formatVersion 1, rejects a non-integer or < 1 ("must be a positive integer") and a newer format with lich's wording ("theme format 2 is newer than this lich reads (up to 1); update lich").
• template.json unchanged: nothing in this repo's tests or CI requires the key, and lich stamps it on install.
• lich#600: the repository rules gain the optional manifest fields minLichVersion (MAJOR.MINOR.PATCH, older lich refuses, dev builds skip; suggested when a pack uses a token only a recent lich has) and formatVersion (same rule as a theme file, error prefixed manifest: ). validate.mjs checks their shape; whether the running lich is new enough stays lich's call.
• CHANGELOG [Unreleased] entries.

Test plan

• validate.mjs on a template-derived theme with formatVersion absent, 1 (ok), 2, 0, 1.5, "1" (rejected with the expected messages)
• validate.mjs on a repository with manifest minLichVersion absent / 0.51.0 (ok), 0.51 / 51 (rejected), manifest formatVersion 2 / "1" (rejected, manifest: prefix)
• node --test tests/*.test.mjs (hook contract tests green in CI)
• fixtures match lich fails on this PR for an unrelated reason: upstream session-start.jsonl gained a line since v0.12.0; left for its own change, since the scripts have to match it first