feat: codebase-analysis skill

8-phase methodology for extracting architectural conventions from
open source repositories:

1. Clone & Shape — measure repo dimensions
2. Import Hierarchy — find foundational packages
3. Interface Contracts — key abstractions
4. Error Handling & Quality Markers — TODOs, style
5. Unique Patterns — project-specific infrastructure
6. Git Archaeology — trace WHY decisions were made
7. PR Discussions — read the actual debates
8. Synthesis — produce analysis.md + conventions.md

Includes pattern-breaks reference (4 categories of why
conventions are violated, with real examples from CockroachDB,
Prometheus, Temporal, Ecto, Oban).

Output: Gitea repos named <project>-conventions.

references/pattern-breaks.md

@@ -0,0 +1,97 @@
+# Pattern Breaks: Why Conventions Are Violated
+
+Based on git archaeology across CockroachDB, Prometheus, Temporal,
+Ecto, and Oban. Each category with real examples.
+
+---
+
+## Category 1: Ship Behavior, Fix Plumbing Later
+
+**Example:** CockroachDB bare goroutine in kvadmission
+
+```go
+// TODO(irfansharif): Use a stopper here instead.
+go func() {
+    ticker := time.NewTicker(10 * time.Minute)
+    for { select { case <-ticker.C: ... } }
+}()
+```
+
+**History:** Irfan Sharif, Aug 2022. Part of the elastic CPU limiter
+(20+ file change). Complex new admission control system. The author
+tagged it with TODO in the SAME commit. Ship the behavior, fix the
+plumbing later.
+
+**Why it survives:** Nobody touched the function for any other reason.
+The goroutine leaks nowhere (process-lifetime). The TODO is correct
+but not urgent.
+
+**For review:** Worth noting, not worth blocking the PR.
+
+---
+
+## Category 2: Better Tooling Exposed the Limitation
+
+**Example:** CockroachDB Handle replacing RunAsyncTask
+
+**History:** Feb 2025, Tobias Grieger. RunAsyncTask was *correct* but
+made every goroutine look identical in execution traces. The Handle
+pattern was motivated by profiling/observability, not correctness.
+
+**Key quote from PR:** "It becomes an ordeal to search for a specific
+goroutine, since all goroutines started by the Stopper get assigned
+the same searchable identity."
+
+**For review:** Track it. This is how progress happens.
+
+---
+
+## Category 3: Removal Cost > Carrying Cost
+
+**Example:** CockroachDB stale "Remove in 22.2" TODO
+
+```go
+// TODO(ajwerner): Remove in 22.2.
+SystemConfigProvider config.SystemConfigProvider
+```
+
+**History:** Feb 2022. Bridge for mixed-version clusters during
+gossip→span-config migration. A struct field that costs one pointer
+of memory. Removing it means touching StoreConfig initialization
+across dozens of call sites.
+
+**Why it survives:** Cost of removal >> cost of carrying. The
+interest rate on this debt is zero.
+
+**For review:** Leave it alone.
+
+---
+
+## Category 4: Context Needs Different Pattern
+
+**Example:** Prometheus global vars for scrape timestamp tolerance
+
+```go
+var ScrapeTimestampTolerance = 2 * time.Millisecond
+```
+
+**History:** Started as "experimental, hidden flag" (Oct 2020). Made
+public after production showed 10ms tolerance saves real disk space.
+
+**Why a global:** Set once at startup from a command-line flag. Read
+thousands of times per second in the scrape loop. Threading a config
+struct would add parameters to every hot-path function for a value
+that never changes after init.
+
+**For review:** Approve it. This IS the right pattern for this context.
+
+---
+
+## Summary Table
+
+| Category | Response in Review | Example |
+|----------|-------------------|---------|
+| Time pressure | Note, don't block | kvadmission goroutine |
+| Better tooling | Track migration | Handle API |
+| Zero-interest debt | Leave alone | Stale version TODOs |
+| Different context | Approve | Hot-path globals |