feat(browser): remove silent html truncation, add --as json

[jackwener]

Summary

Per #opencli-browser discussion (follow-up to #1100), browser get html had two agent-hostile defaults:

1. Silent 50000-char truncation. Agents got a half-DOM with no signal. On any page larger than ~50KB HTML (basically all of them), extraction logic silently saw partial data.
2. Raw HTML string only. Agents had to re-parse to extract structure.

Changes

Behavior	Before	After
Default output	outerHTML.slice(0, 50000), no marker	Full outerHTML, no cap
Opt-in cap	—	--max <n>; truncation prepends <!-- opencli: truncated N of M chars ... -->
Structured output	—	--as json → {selector, matched, tree} tree of {tag, attrs, text, children}
Selector mismatch	stdout (empty) (indistinguishable from empty element)	structured {error:{code:\"selector_not_found\"}} + exit 1
Bad --as / --max	—	invalid_format / invalid_max error codes

--as json shape

{
  \"selector\": \".hero\",
  \"matched\": 1,
  \"tree\": {
    \"tag\": \"div\",
    \"attrs\": { \"class\": \"hero\", \"id\": \"x\" },
    \"text\": \"Hello\",
    \"children\": [
      { \"tag\": \"span\", \"attrs\": {}, \"text\": \"world\", \"children\": [] }
    ]
  }
}

• matched is the full querySelectorAll count, not just 1 — agents know when more elements exist
• text is direct text-children concatenated + whitespace-collapsed; element children carry their own text, ordering between text and elements is not preserved (agents who need ordering should use raw HTML mode)
• attrs pass through untouched

Code layout

• New src/browser/html-tree.ts — the tree JS expression, unit-tested against a DOM stub
• src/cli.ts — command rewrite, structured error paths shared with browser network

Test plan

• src/browser/html-tree.test.ts — serializer on simple elements, whitespace collapse in direct text, recursion with attrs, multi-match first-wins with matched count, zero-match null tree (5 tests)
• src/cli.test.ts browser get html suite — full default, --max with truncation marker, negative --max, --as json envelope, selector_not_found (raw and json), bad --as format (7 tests)
• npm test — 221 files, 1661 tests, 2 skipped, all green
• npm run build — clean TypeScript build + manifest