fix(reporter): contain JSONReporter writer panics as transport failures

[bketelsen]

Summary

JSONReporter.emit recovered a panic from the underlying io.Writer during
the primary json.Encoder.Encode call, but mislabeled it as a Details
encoding failure and routed it through the encoding-error fallback — and the
fallback's own Encode call had no panic recovery at all, so a writer panic
there could escape emit() and crash the caller.

Adds writerPanicGuard, a thin io.Writer wrapper installed around the
writer at NewJSONReporter construction, that recovers a panic from Write
and converts it into an ordinary error before it ever reaches the encoder.
Both the primary and fallback encode paths already treat a returned writer
error correctly (discard it, latch failed, drop later events), so the
writer-panic case now flows through that same existing, tested path instead
of a separate one. A panicking Details json.Marshaler is unaffected: that
panic happens during in-memory marshaling, before Write is ever called, so
encodeRecover still owns it.

Checks

• make check — gofmt leaves no diff, golangci-lint run
  (.golangci.yml) clean, go test -v ./... green (unit + tests/e2e)
• go test -race -short ./... green
• go.mod still declares no dependencies (stdlib only)
• New or changed behavior has focused tests, including failure paths;
  exact output changes update docs/specs/reporter-package.md

Docs housekeeping

• AGENTS.md, docs/design/overview.md, docs/specs/* updated for
  behavior or convention changes (docs/specs/reporter-package.md)
• New docs started from their category's TEMPLATE.md and indexed in
  docs/README.md
• New significant decision recorded as an ADR first, in this PR
• Conformance aliases (ADR-0001) untouched — canonical targets edited
  instead

Protected boundaries

N/A — no .github/workflows/**, .goreleaser.yaml, .svu.yaml, or
policies/agent-governance.json touched.

Verification

• node scripts/check-docs.mjs green:

  ok   docs_index_coverage: 1.000 (required 1)
  ok   link_integrity: 1.000 (required 1)
  ok   symlink_resolution: 1.000 (required 1)
  checked: 12 docs, 153 links, 9 symlinks
  

• Checked against the
  PR review rubric

Risk classification

Moderate. This changes the failure-handling behavior of a shipped public
constructor (NewJSONReporter): a writer panic is now contained and reported
identically to a writer error instead of being mislabeled or, on the
fallback path, potentially escaping to the caller. No public API signature
changed, no protected boundary touched, and the change is narrowly scoped to
reporter/json.go with focused new tests covering both the primary and
fallback panic paths.