fix(validate): plain-text output has no leading pass/fail summary line

Summary

syscribe validate's default (non---json) text output has no leading pass/fail summary line. A clean run prints nothing; a warnings-only run jumps straight into Warnings (N): with no corresponding Errors (0) header ever printed. Answering "did this pass" from the plain-text mode means either noticing the absence of an Errors section (a fragile negative inference) or switching to --json and inspecting severities programmatically.

Motivation

Running syscribe validate after a batch of spec edits, the actual output was:

Warnings (51):

| Code | File | Message |
...

with no errors section at all — meaning it passed (0 errors), but the only way to confirm that from the text output was grep -c "^| E" for an empty result, since there's no explicit "0 errors" anywhere to read. --json solves this cleanly (structured severities), but the plain-text/human-facing mode — which is also what an LLM agent reads by default unless it specifically remembers to pass --json — leaves "did it pass" as an inference rather than a stated fact.

Proposed fix

Print a one-line summary banner at the very top of the default text output, always, regardless of whether there are findings:

0 errors, 51 warnings

or, when gated:

0 errors, 51 warnings (3 gated by --deny W308) — FAIL

Doesn't change --json's shape (already fine) or the exit-code contract — purely a legibility addition to the human/agent-default text path so "did this pass" is answerable by reading the first line, not by the absence of a section.

Acceptance criteria

• A 0-error, 0-warning run prints an explicit 0 errors, 0 warnings line (not silence)
• A warnings-only run's summary line appears before the Warnings (N): table, not after
• The summary reflects gating flags (--deny/--max-warnings/--warnings-as-errors/--profile) when they change the effective pass/fail verdict
• --json output is unchanged

Start at the syscribe validate command's default text-output path and run clean, warnings-only, and gated validation cases. Add the leading summary without changing --json or exit-code behavior; done means the summary always appears first and reflects effective pass/fail status.