feat(knowledge): a run can record what it learned

[EdmondDantes]

What was missing

A run could search and read the knowledge base but had no way to WRITE to it. Nothing taught recording — the tool's description was read-only, no prompt said "record decisions here", and zero notes exist in any project base (only the scaffolded README). The learning a run produced died with the run.

The fix (the action's existence is the teaching)

• KnowledgeWriterInterface — the write half, its OWN interface, exactly as KnowledgeBaseInterface already promised ("Writing lives on its own interface and is obtained separately, so a palette that must not write cannot be handed a thing that writes"). One method: record(kind, title, body, tags).
• KnowledgeBase implements it — files a markdown note into the kind's folder (decision→decisions/, postmortem→postmortems/, design→design/), with frontmatter tags and an H1 title. The kind→folder map lives here because "where things live" is this class's sole policy. The note is readable immediately; searchable from the next indexing pass (the same read-beats-search lag read() already documents).
• KnowledgeTool gains record — but ONLY when constructed with a writer: the action is absent from the schema, the description, and the risk level (Safe without a writer, Mutating with) when there is none. A read-only palette is not told writing exists, rather than told not to. IssueRunner passes the base as both reader and writer, so a run both consults and records.

Correctness details (a clean-context review pass caught these; fixed and tested here):

• A colliding title files a NEW note beside the old (slug-2.md), never over it — by the base's own conventions a decision that replaces an earlier one is a new note. The claim uses an exclusive create (fopen(..., 'x')), not check-then-write, so it holds under the concurrent runs of one project the base is explicitly designed for.
• The title is prepended only when the body has no # H1 of its own — a body opening with ## Context still gets its title, instead of the parser falling back to the filename slug (which rides into every embedded chunk).
• Tags are stripped of the ,[]/newline characters that delimit the frontmatter list, so a stray one cannot mis-file the note.
• The generated handle admits only [a-z0-9-], so ../../outside files as design/outside.md — traversal is not expressible.