feat: wire state backends into all squad operations, not just watch #1003

Problem

The state backend feature (git-notes, orphan-branch, worktree) is currently only wired into the \watch\ command. The
esolveStateBackend()\ function exists in the SDK but is only consumed by \watch/config.ts. All other squad operations (init, agent history reads/writes, decisions, skills) use \FSStorageProvider\ directly.

This means setting \stateBackend: 'git-notes'\ in config.json only affects watch — not the broader squad lifecycle.

Solution (phased)

Phase 1 (this issue) ✅

• Extend \StateBackend\ interface with \delete()\ and \�ppend()\
• Add \SquadStateContext\ and
  esolveSquadState()\ factory to resolution module
• Add comprehensive docs for state backends
• See PR Merge dev to main: Publish docs site with updated messaging #997

Phase 2 (follow-up)

• Migrate \squad init\ to use \SquadStateContext\ when writing initial state files
• Migrate agent history reads/writes to go through the backend
• Migrate decisions inbox merge (Scribe) to use the backend

Phase 3 (follow-up)

• Migrate skills, casting, and remaining modules
• Add caching layer for git-notes (currently loads full JSON blob on every read)
• Address concurrency (optimistic retry for git-notes, locking for orphan)

Design decisions

• Boundary: Bootstrap artifacts (config.json, team.md structure checks) stay on local filesystem. Only mutable squad state flows through the backend.
• No adapter pattern: We do NOT wrap \StateBackend\ as a \StorageProvider. The interfaces have different semantics (5 vs 24 methods). Instead, commands resolve the backend once and use it directly.
• Incremental migration: Commands adopt \SquadStateContext\ one at a time in follow-up PRs.

Related: #995 (docs)