fix: surface .lashignore and explain the linter's own codes (#58)

[fohara]

Closes #58.

Two discoverability failures from the report, both ending at the same place: the user is looking at a diagnostic and the thing that resolves it is documented somewhere they are not.

.lashignore

It already worked — honoured by both walkers, tested, used by this repo — and was reachable from nothing a user reads when a content/ directory starts reporting W_INDEX_ORPHAN once per file.

• The warning message names it: ... is not referenced in the root index (add it to .lashignore if it is not a task file). The pointer is in the message rather than only the help field because help surfaces only under -v, and one warning per file is exactly when the escape hatch has to be on screen.
• lash lint --help and the top-level lash --help describe file discovery (.gitignore + .lashignore).
• README, docs/user-guide.md and docs/error-codes.md each document it.

lash explain

lash explain W_INDEX_ORPHAN and lash explain E_LINK_NOT_FOUND answered "Unknown error code" — the linter's per-rule codes were never connected to the explanation table (the E_LINT_* entries predate them).

• All 29 missing codes now have entries, including E_SEM_DESC_TOO_LONG and E_NOTE_EXCESSIVE_LENGTH, which only appear at a different severity and so are invisible to a rule's code(). Also added generic E_PARSE, which lint emits directly.
• A registry test walks register_default_rules and fails if any rule's code has no explanation, so a new rule cannot reintroduce the gap.
• explain --list was silently dropping every code its if-else prefix chain did not match — all W_ and I_ codes. Replaced with a prefix table plus an explicit "Other Codes" fallback. Order matters: E_INDEX_FILE_MISSING is a cross-file rule, not a database error.
• Warnings and info codes are labelled Warning: / Info: instead of Error:.
• Lint's summary now names one of the codes it reported and the lash explain invocation for it — picking a code explain actually knows, so the hint can't create the dead end it exists to prevent.

error_explanations.rs was split into a module directory (parse / syntax / semantic / crossfile / runtime / creation, mirroring the --list grouping); it was already past the repo's 500-line guideline and this change nearly doubled it.

Verification

Reproduced the report end to end: content/a-post.md warns with the .lashignore pointer, printf 'content/\n' > .lashignore clears it, and lash explain W_INDEX_ORPHAN now answers. Full suite, doctests, clippy -D warnings and cargo fmt --check pass; lint/agent-prompt snapshots updated for the new message and footer.