Add ecosystem evaluation docs: Polytician ↔ AgentVault integration status

[johnnyclem]

Summary

This repo (johnnyclem/polytician) sits in a broader ecosystem alongside AgentVault,
SmallChat, Stenographer, and Short-Hand, each evaluated from its own repo since no single
session can browse all of them. This PR adds Polytician's side of that evaluation.

• Adds docs/ecosystem/executive-summary.md and docs/ecosystem/engineering-guide.md,
  matching the structure of AgentVault's own docs/ecosystem/* docs so all repos are
  cross-linkable.
• Cross-checks AGENTVAULT_COMPATIBILITY_PRD.md (a 7-task spec in this repo describing what
  AgentVault needs to build to support Polytician) against AgentVault's actual main branch
  by fetching the exact files it names via raw GitHub URLs (this session's API scope is
  polytician-only, so no shell/API access to AgentVault was available — everything about
  AgentVault is sourced from specific file fetches, flagged [AV-verified] in the docs).
• Key finding: 4 of the PRD's 7 tasks are already shipped on AgentVault's side (REST
  bridge routes, the Polytician config parser, orchestrator context enrichment, CLI
  subcommands), 2 are confirmed not started (canister-level MCP server registry, webapp
  dashboard UI), and 1 (AgentVault's test coverage) couldn't be checked from this side. The
  PRD currently reads as a from-scratch backlog; it should be updated so contributors don't
  redo already-shipped work.
• Confirms AgentVault's own engineering guide is factually correct about Polytician: it
  references src/orchestration/polytician-enricher.ts (which is real and matches this
  repo's expected search_concepts/read_concept MCP contract) and correctly flags its
  hard character-count truncation as the reason to eventually route through Short-Hand's
  CompactionEngine — no changes needed on Polytician's side for that future migration.
• Links the new docs, the PRD, and the PolyVault spec from README.md's new "Ecosystem"
  section — previously the README made no mention of the AgentVault/PolyVault integration
  at all, despite it accounting for roughly a third of src/ and docs/.

Test plan

• Docs-only change; no code touched. npm run typecheck / npm test not required but
  unaffected.
• Verified all internal markdown links resolve to files present in this PR/repo.
• Verified every AgentVault-side claim against a specific fetched file (see tables in
  docs/ecosystem/engineering-guide.md) rather than repeating unverified README claims.

---

Generated by Claude Code