docs: add complete reference docs.md (replaces internal api.clawagent.sh draft)

The previous docs.md was an internal-only API reference for the
hand-rolled api.clawagent.sh deployment — full of customer URLs and
hosted-endpoint specifics. It was gitignored back in the
chore/oss-cleanup-public sweep.

This commit replaces it with a complete public-facing reference and
un-gitignores the path.

Contents:
  - Install + quickstart
  - Core API: ComputerAgent, ChatHandle, runTask (with full options
    table cross-checked against ComputerAgentOptions in packages/sdk/
    src/types.ts)
  - IdentitySource: git / local / inline + string shorthand
  - Substrates: Local / Bwrap / E2B / VZ with package + startup notes
  - Engines: claude-agent-sdk / gitagent / deepagents + custom path
  - Session stores: memory / file / mongo / sqlite + resume pattern
  - Telemetry: AgentTelemetry (library-mode) + AuditSink (server-mode)
  - Permissions: onToolCall, ttyApproval, manual iteration
  - Policy: SRS-backed Cedar + OPA guardrails
  - Configuration: env var matrix, model override resolution order
  - HTTP wire protocol: REST endpoints + SSE event union + curl example
  - CLI flag table
  - Typed errors
  - Companion package map
  - Versioning policy

Sections cross-link to README.md, packages/computeragent/info.md
(architecture), CHANGELOG, CONTRIBUTING, SECURITY.

Audit: 0 customer-name leaks (lyzr/nordstrom/clawagent/shreyas-lyzr).
560 lines.

Author: shreyas-lyzr

.gitignore

@@ -54,4 +54,3 @@ PRIVATE.md
 
 # Internal planning / handoff docs that should not be on the public repo
 PLAN.md
-docs.md