feat: align agent stacks (Claude, Cursor) on shared bootstrap

Unifies coding-agent setup across Claude Code and Cursor so both stacks get:
- Pin-matched bd from flake.nix
- Fresh Dolt beads tracker sync (not stale JSONL export)
- Same skills via symlinks to .agents/skills/

Key changes:

1. scripts/agent-bootstrap.sh: stack-agnostic bootstrap that handles bd
   installation, tracker sync, and optional cargo warming. Works for any
   agent stack (Claude, Cursor, or manual).

2. .claude/hooks/session-start.sh: now a thin wrapper that calls the
   shared bootstrap for cloud sessions, keeping only the local pin-check
   inline.

3. .cursor/environment.json: configures Cursor cloud agents to run the
   bootstrap via 'install'.

4. .cursor/skills/: symlinks to .agents/skills/* so Cursor agents
   discover the same skills Claude has.

5. AGENTS.md: adds Multi-Stack Agent Support section; clarifies that
   scripts/kb is the KB interface (Prolog MCP is not required and not
   available in all stacks).

6. context/cloud-agents.md: updated for multi-stack support; added KB
   section emphasizing scripts/kb over Prolog MCP.

The canonical skill source remains .agents/skills/. Both .claude/skills/
and .cursor/skills/ are symlinks to it, so there is one truth that both
stacks read.

Co-authored-by: Ed Langley <fiddlerwoaroof@users.noreply.github.com>

.cursor/environment.json

@@ -0,0 +1,5 @@
+{
+  "name": "Improvise",
+  "install": "./scripts/agent-bootstrap.sh --skip-cargo-warm",
+  "start": "nix-daemon &"
+}

.cursor/skills/beads

@@ -0,0 +1 @@
+../../.agents/skills/beads

.cursor/skills/beads-merge

@@ -0,0 +1 @@
+../../.agents/skills/beads-merge

.cursor/skills/code-review

@@ -0,0 +1 @@
+../../.agents/skills/code-review

.cursor/skills/kb

@@ -0,0 +1 @@
+../../.agents/skills/kb

.cursor/skills/pr-watch

@@ -0,0 +1 @@
+../../.agents/skills/pr-watch

.cursor/skills/wave

@@ -0,0 +1 @@
+../../.agents/skills/wave