fix(sdk): state backend hardening — retry, circuit-breaker, startup verification

[diberry]

What

Add retry with exponential backoff, circuit-breaker pattern, and startup verification to state backends.

Why

State backends silently swallow git errors (resolveStateBackend catches and ignores, external backend falls back silently). This causes silent data loss and makes debugging impossible.

How

1. Retry transient git errors with exponential backoff (3 retries, 100ms base, 2s max)
2. Circuit-breaker trips after 5 consecutive failures, fast-fails subsequent ops with 30s cooldown
3. Startup health check — read-only verifyStateBackend() validates backend accessibility
4. Error surfacing — all errors logged with context; explicit backend selection throws on failure instead of silent fallback
5. execSync → execFileSync migration for write paths (eliminates shell injection surface)

Closes #864
Related: #895 (Phase 1, items #4, #5)

[github-actions]

🟡 Impact Analysis — PR #907

Risk tier: 🟡 MEDIUM

📊 Summary

Metric	Count
Files changed	4
Files added	1
Files modified	3
Files deleted	0
Modules touched	3
Critical files	1

🎯 Risk Factors

• 4 files changed (≤5 → LOW)
• 3 modules touched (2-4 → MEDIUM)
• Critical files touched: packages/squad-sdk/src/index.ts

📦 Modules Affected

root (1 file)

• .changeset/state-backend-hardening.md

squad-sdk (2 files)

• packages/squad-sdk/src/index.ts
• packages/squad-sdk/src/state-backend.ts

tests (1 file)

• test/state-backend.test.ts

⚠️ Critical Files

• packages/squad-sdk/src/index.ts

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This report is generated automatically for every PR. See #733 for details.

[github-actions]

🏗️ Architectural Review

⚠️ Architectural review: 1 warning(s).

Severity	Category	Finding	Files
🟡 warning	export-surface	Package entry point(s) modified with 4 new/changed export(s). New public API surface requires careful review for backward compatibility.	packages/squad-sdk/src/index.ts

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Automated architectural review — informational only.

[Copilot]

Pull request overview

This PR hardens the SDK’s git-based state backends by adding retry/backoff and a circuit-breaker, surfacing previously swallowed config/initialization errors, and introducing a read-only startup verification helper.

Changes:

• Add retry helpers (exponential backoff) and a CircuitBreaker applied to git-backed backend operations.
• Make resolveStateBackend() warn on config read failures and fail-fast for explicit backend selections that can’t initialize.
• Export new APIs (CircuitBreaker, verifyStateBackend) and add/extend tests + changeset.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
packages/squad-sdk/src/state-backend.ts	Implements retry/backoff, circuit breaker, config warning/fail-fast behavior, and verifyStateBackend(); migrates write paths to execFileSync.
packages/squad-sdk/src/index.ts	Exports CircuitBreaker and verifyStateBackend from the SDK barrel.
test/state-backend.test.ts	Adds tests for warnings, circuit breaker behavior, and backend verification.
.changeset/state-backend-hardening.md	Adds a patch changeset documenting the hardening work.

[diberry]

🔍 Squad Review — Kaylee (Engineering)

#	Check	Status	Notes
1	Changelog entry	✅	.changeset/state-backend-hardening.md (SDK patch)
2	Squashed to 1 commit	✅	1 commit
3	CI green	✅	All checks passed
4	Copilot comments resolved	✅	5/5 threads resolved with fixes or acknowledgments
5	No .squad/ files	✅	Clean
6	No unrelated files	✅	4 files — all scoped to state backend hardening
7	Tests for changes	✅	est/state-backend.test.ts updated
8	Not a duplicate/reversal	✅	Unique scope (closes #864); #916 is separate (shell injection)

Verdict: ✅ Ready to merge

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Review by Squad AI team (Kaylee — Engineering) · requested by Dina Berry