Add circular redirect detection to linter

[jamengual]

Summary

• Circular redirect detection in the linter via graph-based DFS cycle detection — catches direct cycles, transitive chains, prefix self-loops, cross-host cycles, and regex/glob sample-based tracing
• Enriched webhook response — lint failures return HTTP 422 with structured JSON containing all issues instead of a generic "Sync failed", enabling CI pipelines to parse and surface specific feedback
• Comprehensive documentation — detection types, worked examples with colored output, JSON/CI usage, webhook response format, and limitations table

What's Changed

Lint: Circular Redirect Detection (internal/lint/lint.go)

New checkCircularRedirects() builds a directed graph from redirect rules and runs DFS with three-color marking to find cycles:

Detection	Severity	Example
Direct cycle (A→B→A)	Error	/a → /b, /b → /a
Transitive cycle (A→B→C→A)	Error	/a → /b → /c → /a
Prefix cross-cycle	Error	/foo/ → /bar/, /bar/ → /foo/
Cross-host cycle	Error	example.com/p1 → example.com/p2 → …/p1
Prefix self-loop (preserve_path)	Error	/old/ → /old/new/ expands infinitely
Regex self-match	Warning	^/api/v1/(.*) → /api/v1/v2/$1 (sample-based)
Glob loop	Warning	Glob destination falls within same match (sample-based)

Supporting functions: buildRedirectGraph, findCycles, extractDestinationPath, checkPrefixSelfLoops, checkRegexGlobCycles, generateRegexSamples/generateGlobSamples.

Webhook: Structured Lint Error Response (cmd/redirector-sync/main.go)

• New LintError type carries the full lint.Result
• SyncOnce() returns *LintError on lint failures (instead of fmt.Errorf)
• Webhook handler uses errors.As to detect lint failures and returns HTTP 422 with JSON:

{
  "status": "lint_failed",
  "source": "github-primary",
  "message": "lint errors found in config from source github-primary (1 errors)",
  "issues": [
    {
      "severity": "error",
      "rule_id": "old-home",
      "message": "Circular redirect detected: old-home -> new-home -> old-home",
      "suggestion": "Remove one rule from the chain or change a destination to break the cycle"
    }
  ]
}

Documentation

• docs/SYNCER.md — New "Circular Redirect Detection" section with graph diagram, detection table, 4 worked examples (direct cycle, prefix self-loop, regex warning, JSON for CI), webhook response format, and limitations table
• docs/FEATURES.md — Added circular redirect detection to lint checks list
• docs/MANAGEMENT_API.md — Documented webhook response format (success/422/500)
• IMPLEMENTATION_PLAN.md — Phase 9 plan with Stage 9.1 marked complete

Tests (internal/lint/lint_test.go)

10 new tests covering all detection scenarios:

• TestLinter_CheckCircularRedirects_DirectCycle
• TestLinter_CheckCircularRedirects_TransitiveCycle
• TestLinter_CheckCircularRedirects_NoCycle
• TestLinter_CheckCircularRedirects_ExternalDestination
• TestLinter_CheckCircularRedirects_CrossHostCycle
• TestLinter_CheckCircularRedirects_PrefixSelfLoop
• TestLinter_CheckCircularRedirects_PrefixNoSelfLoop
• TestLinter_CheckCircularRedirects_NonRedirectSkipped
• TestLinter_CheckCircularRedirects_PrefixCrossCycle
• TestLinter_CheckCircularRedirects_RegexSelfCycle

Test plan

• All 10 circular redirect detection tests pass
• All 25 lint tests pass (no regressions)
• Full project test suite passes (go test ./...)
• golangci-lint run ./internal/lint/ — 0 issues
• go build ./... — compiles clean
• Manual test: create config with circular redirect, run redirector-sync --lint --lint-config config.yaml
• Manual test: trigger webhook with circular config, verify HTTP 422 response

🤖 Generated with Claude Code