docs(auth): add docs/AUTH.md — single source of truth (KLA-410)

[jklaassenjc]

Summary

Closes KLA-410.

Adds docs/AUTH.md — the doc the security feedback called out as missing. It pulls together how jc authenticates, what it does and does not enforce on its own, how the MCP server's trust model varies by transport, what the audit log captures, and what the threat model looks like per scenario. Links from the existing Authentication Methods section in the README.

The doc is honest about gaps that are tracked but not yet shipped:

• API key is still the default (KLA-407)
• The MCP execute: true gate is agent-controlled with no human-in-the-loop or signed identity (KLA-408)
• Per-operator identity is not captured in the audit log (KLA-408)
• Profile-level read-only enforcement isn't wired (KLA-409)

Every code-anchored claim cites a internal/.../*.go:line location so reviewers can verify each statement against the source.

Notable corrections vs. the original audit

While verifying facts against the code, two things from the earlier audit summary turned out to be wrong and are stated correctly in the doc:

1. MCP HTTP/SSE binds to 127.0.0.1 by default, not all interfaces. There's an explicit warning when binding non-loopback without TLS (internal/cmd/mcp.go:218).
2. 30 destructive MCP tools use the destructiveInput pattern, not 17.

Test plan

• go build ./... clean
• go test ./internal/cmd/... ./internal/config/... ./internal/api/... ./internal/mcp/... passing
• wc -l docs/AUTH.md → 355 lines (acceptance: ≤600)
• All four user-feedback points have a directly-quotable answer in the doc

🤖 Generated with Claude Code

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Note

Low Risk
Documentation-only change: adds an auth/authorization and MCP security model reference with no runtime or behavior changes.

Overview
Adds a new docs/AUTH.md as a single source of truth for jc’s authentication methods, credential storage/precedence, authorization limitations, destructive-operation gating (CLI confirmation + MCP execute: true), MCP transport trust model, and MCP audit log shape/redaction.

Updates README.md to link to docs/AUTH.md from the Authentication Methods section so security reviewers have one canonical reference.

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