docs: migrate agent coordination from MCP Agent Mail to Liquid Mail

Replace all MCP Agent Mail references with Liquid Mail in AGENTS.md and
CLAUDE.md. The old system used file reservations and MCP-based messaging
with inbox/outbox/thread semantics. Liquid Mail provides a simpler
post-based shared log with topic-scoped messages, decision conflict
detection, and polling via the liquid-mail CLI.

Key changes:
- Remove entire MCP Agent Mail section (identity registration, file
  reservations, macros vs granular tools, common pitfalls)
- Update Beads integration workflow to reference Liquid Mail: replace
  reservation + announce patterns with post-based progress logging and
  decision-first workflows
- Update bv scope boundary note to reference Liquid Mail
- Append full Liquid Mail integration block to CLAUDE.md: conventions,
  typical flow, decision conflicts, posting format, topic rules, context
  refresh, live updates, mapping cheat-sheet, quick reference
- Add .liquid-mail.toml project configuration (Honcho backend)

.liquid-mail.toml

@@ -0,0 +1,13 @@
+# Liquid Mail config (TOML)
+#
+# Prefer env vars for secrets:
+#   LIQUID_MAIL_HONCHO_API_KEY
+#   LIQUID_MAIL_HONCHO_WORKSPACE_ID
+#
+[honcho]
+api_key = "hch-v3-pmx23gk9k60xlqffpxpyjj8pywnxkpjkic9bdygx21iydvyxbeialioz5ehhcp1r"
+# workspace_id is optional.
+# If omitted, Liquid Mail defaults it to the repo name (git root folder name).
+# Honcho uses get-or-create semantics for workspaces, so it will be created on first use.
+# workspace_id = "my-repo"
+base_url = "https://api.honcho.dev"