feat(swarm): node identity bootstrap + node_identity_get tool (#76)

[Dewinator]

Closes #76 — Swarm Phase 1b.

Summary

• New scripts/init-node-identity.mjs: idempotent Ed25519 keypair bootstrap. Privkey lands in ~/.mycelium/node.key (mode 0600, 0700 directory); pubkey + derived node_id go into the nodes self-row.
• New MCP tool node_identity_get: read-only, returns { node_id, pubkey_b64, display_name, created_at }.
• node_id follows docs/SWARM_SPEC.md §3.5 exactly: base58btc(multihash(sha2-256, pubkey_raw)) → 46-char Qm… string.

Research summary

• Inspected the existing MCP-tool pattern (tools/identity.ts, tools/soul.ts) and copied the read-only getSelfModel shape for the new tool.
• node:crypto already supports Ed25519 (generateKeyPairSync('ed25519')); existing services/crypto.ts uses the same SPKI-tail pattern to extract the raw 32-byte pubkey, so I reused it.
• The wire-format formula was already pinned in docs/SWARM_SPEC.md (merged in a50444a); the migration spec from Swarm Phase 1a: migration 070 — node_identity table (file only, do NOT run) #75 is in supabase/migrations/070_node_identity.sql (merged in 3cce198). PR feat(swarm): migration 070 — nodes table for cryptographic identity (#75) #79 (which includes the contract test) is still open but the schema needed by this PR is already on main.
• Encoded base58btc by hand (no extra dep) with explicit leading-zero handling — pinned by tests against the canonical 2NEpo7TZRRrLZSi2U 'Hello World!' vector.

What this does NOT do

• No SQL is executed. Migration 070 must be applied to the database manually before the bootstrap script can live-run; the script will fail with a clear PostgREST error if the nodes table is missing.
• No display_name heuristic — the bootstrap optionally honours MYCELIUM_NODE_DISPLAY_NAME, otherwise leaves the field NULL.
• No automatic call from setup.sh/install.sh — wiring the bootstrap into the installer is a follow-up.
• No use of the new identity by any other tool (advertisements, signed lessons, peer discovery) — those are later swarm phases.

Constitution affirmation

Touches Pillar 1 (Decentralized) and Pillar 6 (Cyber security). Reinforced, not weakened:

• Pillar 1: identity is generated and owned locally; no central registry consulted.
• Pillar 6: the privkey never reaches the database, lives in a chmod-600 file inside a chmod-700 directory, and bootstrap refuses to regenerate when DB state is inconsistent (would otherwise orphan every signature ever produced under the original key).
• No other pillar weakened — pairing/breeding/swarm-economics surfaces untouched.

Test plan

• cd mcp-server && npm test — 271 tests pass, including 12 new ones in node-identity.test.ts:
  • base58btc encoding (empty buffer, leading-zero rule, canonical 'Hello World!' vector)
  • computeNodeId rejects non-32-byte input, is deterministic, distinguishes inputs, produces a 46-char Qm…, matches the spec formula
  • bootstrap idempotency: first run inserts, second run is a no-op, refusal-on-inconsistent-state, deterministic re-derivation from existing privkey
• Live-test (after migration 070 is applied to a real Supabase): cd mcp-server && npm run build && node scripts/init-node-identity.mjs — verify (a) ~/.mycelium/node.key is created with 0600, (b) nodes table has exactly one is_self=true row, (c) running again prints node_id already initialized: … and inserts nothing, (d) calling node_identity_get from any MCP client returns the same node_id.