refactor: Dedupe backend helpers and polish signature caps, markers, clap help

[Xuepoo]

Summary

Polish pack closing six review items:

1. signature() promoted to a Language trait default across ~11 backends (CSS keeps its selector-list override); string_value() deduped into lang/util.rs (Lua keeps its content-field wrapper).
2. Doc-comment attachment now requires direct adjacency (no blank line), depth-2 skip walk removed.
3. Signature caps count chars not bytes — CJK signatures no longer truncate ~4x early; magic token index replaced with a named constant.
4. Collapse markers singularized: [1 line omitted] when n==1 (ctx-exec marker plus three ctx-symbol compact-view sites via shared omit_marker()).
5. Clap UX: --kind help corrected to var; exec subcommand gained after_help explaining the whole command must be one quoted argument (ctxctl exec "cargo test").

Closes CTX-0039.

Testing

• New tests: CJK char-cap behavior, doc-comment adjacency rejection, singular marker, exec help/hint text, kind validation wording.
• Gates: fmt / clippy -D warnings / full workspace tests green (13 suites).