docs: consolidate — bugs/known-issues.md absorbs gateway issues, GATEWAY.md moves to repo-level docs/

Matches the repo's existing doc layout:
  - /docs/ has topic-based guides per subsystem (AUTHENTICATION.md,
    PAYMENT.md, SCHEDULER.md, etc.)
  - /bugs/known-issues.md is the single user-facing known-issues
    registry; the file's own rules say entries are REMOVED on fix
    and new ones added in-place.

This commit aligns gateway docs with that layout:

  /gateway/docs/STORY.md        → /docs/GATEWAY.md
  /gateway/docs/BUGS_AND_...md  → merged into /bugs/known-issues.md
  /gateway/docs/                → deleted

docs/GATEWAY.md: the end-to-end walkthrough. 8 internal links rewritten
for the new location (one level up from where it was):
  ../openapi.yaml    → ../gateway/openapi.yaml
  ../README.md       → ../gateway/README.md
  ../agents/...      → ../gateway/agents/...
  ../recipes/...     → ../gateway/recipes/...
  ../../examples/... → ../examples/...

Link check across the four affected docs (docs/GATEWAY.md,
gateway/README.md, fleet/README.md, bugs/known-issues.md) — all
resolve.

bugs/known-issues.md absorbs the unresolved work the gateway-local
file was tracking. Net:

  - 15 new gateway entries (5 medium, 6 low, 5 nit) with file
    paths, symptoms, workarounds — all the items surfaced during
    the recipes/SSE/DID work that aren't yet fixed.
  - 2 existing entries narrowed: `tool-name-collisions-silent`
    was partially fixed in this session (collision rejection
    landed at plan-open time) and is now
    `parse-agent-from-tool-greedy-mismatch` tracking only the
    residual regex issue. `signature-verification-ok-when-unsigned`
    narrowed to `signature-verification-non-text-parts-unverified`
    — the envelope ambiguity was fixed by the four-valued
    `verified` label, `data`/`file` parts are still unverified.
  - Quick-index table + TOC counts updated accordingly.
  - Header's `_Last updated_` line reflects the 2026-04-20 session.

gateway/README.md + examples/gateway_test_fleet/README.md: 6 total
references to `docs/STORY.md` rewritten to point at the new
`docs/GATEWAY.md` location.

No code changes; 216/216 tests still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: raahulrahl