fix: best-effort fallback when slash line is invalid grammar

[nettapper]

Summary

ParseTask aborted the entire task with a hard error when a line outside a
fenced code block started with / but wasn't a valid slash command (e.g.
//, lone /, /=foo). One bad line in an appended prompt was enough
to discard the entire prompt, the frontmatter, and every sibling task in the
directory.

In-fence slashes were already handled by the markdown-aware splitter; only
the bare-text case was broken.

Also threads a *slog.Logger through the parse entry points so callers can
route the new WARN logs through their own logger instead of slog.Default().

What changed

• pkg/codingcontext/taskparser/markdownparser.go — parseGrammar now
  falls back to a line-by-line parse on failure. Lines that still don't parse
  are emitted as raw text and a WARN is logged with the offending line and
  the underlying grammar error. The whole-segment fast path is unchanged for
  valid input.

• API additions (backward compatible):

  • taskparser.NewExtension(logger *slog.Logger) goldmark.Extender — the
    existing Extension global is now NewExtension(nil).
  • taskparser.ParseTaskWithLogger(text, logger) — ParseTask is a
    nil-logger wrapper.
  • markdown.ParseMarkdownFileWithLogger(path, fm, logger) —
    ParseMarkdownFile is a nil-logger wrapper.

  Nil loggers resolve to slog.Default() at log time (not capture time), so
  slog.SetDefault changes made after package load still take effect.

• pkg/codingcontext/context.go — all five ParseMarkdownFile call
  sites and the one ParseTask call site now pass cc.logger, so parser
  WARNs route through whatever logger was injected via
  codingcontext.WithLogger.

• pkg/codingcontext/taskparser/taskparser_test.go — new coverage:

  • TestParseTask_BestEffortMalformedSlash — three subcases (//, lone
    /, /=foo) outside a code block, each asserting both the malformed
    line and the surrounding text survive (guards against a fix that
    salvages only the bad line).
  • TestParseTaskWithLogger_RoutesWarnToInjectedLogger — captures stderr
    from an injected logger and verifies the WARN lands there.
  • TestParseTaskWithLogger_NilLoggerResolvesLazyDefault — uses
    slog.SetDefault after construction to confirm nil-logger callers get
    the current default, not a snapshot.

Out of scope

• The grammar itself is unchanged. A larger rewrite (e.g. teaching the
  grammar to treat any unrecognized /-prefixed line as text directly) would
  be cleaner but riskier.