Discover slash commands for Kimi and OpenCode (#22)

* fix: dispatch kimi and opencode panes through the reportedAgent fallback

CatalogFor and CatalogForProfile each carried their own agent-name switch, and
the two had drifted: CatalogFor mapped kimi and opencode, the reportedAgent
fallback in CatalogForProfile did not. The relay passes an empty profileID
whenever the agent's own binary is missing from the service PATH - the norm
under launchd's minimal environment - so for those two kinds the fallback found
no provider, execution fell through to discoverGenericSkills(nil, ""), and the
pane got an empty palette: not even builtins. Measured through the server's own
call shape, a kimi pane lost 80 commands and an opencode pane lost 17.

Both entrypoints now resolve through one profileIDForAgentName table, so the
mapping cannot drift again.

The existing TestCatalogForExactDispatch could not catch this, because
CatalogFor supplies the profileID itself and never reaches the fallback. The new
test asserts the shape internal/app/server.go actually produces - an empty
profileID plus a reportedAgent - for every name the dispatch table covers.
Against the previous catalog.go it fails on exactly the seven kimi and opencode
aliases.

* feat: fold YAML block scalars in skill frontmatter

The frontmatter key pattern captured only the remainder of the key's own
line, so a block scalar - description: | - parsed to the literal value "|".
Eleven skills on a developer machine render that way, and a one-character
description also defeats the rule that drops skills without one.

Fold a block scalar's indented continuation lines into the value: literal
blocks join with newlines, folded blocks with spaces, common indentation is
stripped, and the block ends at the next unindented key or the closing fence.
Chomping indicators are recognised but not honoured because every consumer
already collapses whitespace. A malformed block yields an empty value, so the
skill is dropped rather than shown with garbage.

* test: pin that the recursive command walk cannot loop on a symlink

walkDirBudget's safety currently rests on skipping symlinks outright, which
was only stated in a comment. This fixture - a link pointing at an ancestor -
fails against a version that follows symlinked directories without threading
real-path de-duplication through the recursion, so the invariant is now
enforced rather than described.

* fix slash command block frontmatter

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Co-authored-by: Christophe Vidal <christophe.vidal.1983@gmail.com>