feat(dreaming): add Pi AgentSession planning harness

[NicholaiVogel]

Summary

• adds @earendil-works/pi-coding-agent and a Pi-backed AgentSession factory using the daemon’s resolved Pi model and credentials
• makes the Refactor: make agentic dreaming and pipeline dreaming two ways of doing the same thing. #913 ingest planner route Pi providers through an AgentSession, returning only a strict plan body to the existing daemon-owned validation/apply/lease seam
• exposes capability-constrained context inspection and plan submission tools; no tool has direct database or filesystem write access

Relationship

Stacked on #946 (#913 unified ingest) because this consumes its leased context → plan → deterministic apply boundary.

Verification

• bun test platform/daemon/src/pipeline/ingest platform/daemon/src/pipeline/extraction-queue.test.ts --timeout 120000 (101 passing)
• bun run --filter @signet/daemon build (passed)

PR readiness

• Spec alignment validated (INDEX.md + dependencies.yaml)
• Agent scoping verified on all new/changed data queries
• Input/config validation and bounds checks added
• Error handling and fallback paths tested (no silent swallow)
• Security checks applied to admin/mutation endpoints
• Docs updated for API/spec/status changes
• Regression tests added for each bug fix
• Lint/typecheck/tests pass locally

No public API or spec status changed; the documentation readiness item is not applicable to this internal planning-harness change.

[aaf2tbz]

Reviewed by hand: No appearing faults or edge cases. Code looks clean and imports / exports align with their callee values and definitions / aligned functions.