feat(debug): add persistent knowledge base for resolved sessions (#961)

When a debug session resolves, append a structured entry to
.planning/debug/knowledge-base.md capturing error patterns,
root cause, and fix approach.

At the start of each new investigation (Phase 0), the debugger
loads the knowledge base and checks for keyword overlap with
current symptoms. Matches surface as hypothesis candidates to
test first — reducing repeat investigation time for known patterns.

The knowledge base is append-only and project-scoped, so it
builds value over the lifetime of a codebase rather than
resetting each session.

agents/gsd-debugger.md

@@ -733,6 +733,48 @@ Can I observe the behavior directly?
 
 </research_vs_reasoning>
 
+<knowledge_base_protocol>
+
+## Purpose
+
+The knowledge base is a persistent, append-only record of resolved debug sessions. It lets future debugging sessions skip straight to high-probability hypotheses when symptoms match a known pattern.
+
+## File Location
+
+```
+.planning/debug/knowledge-base.md
+```
+
+## Entry Format
+
+Each resolved session appends one entry:
+
+```markdown
+## {slug} — {one-line description}
+- **Date:** {ISO date}
+- **Error patterns:** {comma-separated keywords extracted from symptoms.errors and symptoms.actual}
+- **Root cause:** {from Resolution.root_cause}
+- **Fix:** {from Resolution.fix}
+- **Files changed:** {from Resolution.files_changed}
+---
+```
+
+## When to Read
+
+At the **start of `investigation_loop` Phase 0**, before any file reading or hypothesis formation.
+
+## When to Write
+
+At the **end of `archive_session`**, after the session file is moved to `resolved/` and the fix is confirmed by the user.
+
+## Matching Logic
+
+Matching is keyword overlap, not semantic similarity. Extract nouns and error substrings from `Symptoms.errors` and `Symptoms.actual`. Scan each knowledge base entry's `Error patterns` field for overlapping tokens (case-insensitive, 2+ word overlap = candidate match).
+
+**Important:** A match is a **hypothesis candidate**, not a confirmed diagnosis. Surface it in Current Focus and test it first — but do not skip other hypotheses or assume correctness.
+
+</knowledge_base_protocol>
+
 <debug_file_protocol>
 
 ## File Location
@@ -883,6 +925,16 @@ Gather symptoms through questioning. Update file after EACH answer.
 <step name="investigation_loop">
 **Autonomous investigation. Update file continuously.**
 
+**Phase 0: Check knowledge base**
+- If `.planning/debug/knowledge-base.md` exists, read it
+- Extract keywords from `Symptoms.errors` and `Symptoms.actual` (nouns, error substrings, identifiers)
+- Scan knowledge base entries for 2+ keyword overlap (case-insensitive)
+- If match found:
+  - Note in Current Focus: `known_pattern_candidate: "{matched slug} — {description}"`
+  - Add to Evidence: `found: Knowledge base match on [{keywords}] → Root cause was: {root_cause}. Fix was: {fix}.`
+  - Test this hypothesis FIRST in Phase 2 — but treat it as one hypothesis, not a certainty
+- If no match: proceed normally
+
 **Phase 1: Initial evidence gathering**
 - Update Current Focus with "gathering initial evidence"
 - If errors exist, search codebase for error text
@@ -1056,6 +1108,37 @@ Then commit planning docs via CLI (respects `commit_docs` config automatically):
 node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" commit "docs: resolve debug {slug}" --files .planning/debug/resolved/{slug}.md
 ```
 
+**Append to knowledge base:**
+
+Read `.planning/debug/resolved/{slug}.md` to extract final `Resolution` values. Then append to `.planning/debug/knowledge-base.md` (create file with header if it doesn't exist):
+
+If creating for the first time, write this header first:
+```markdown
+# GSD Debug Knowledge Base
+
+Resolved debug sessions. Used by `gsd-debugger` to surface known-pattern hypotheses at the start of new investigations.
+
+---
+
+```
+
+Then append the entry:
+```markdown
+## {slug} — {one-line description of the bug}
+- **Date:** {ISO date}
+- **Error patterns:** {comma-separated keywords from Symptoms.errors + Symptoms.actual}
+- **Root cause:** {Resolution.root_cause}
+- **Fix:** {Resolution.fix}
+- **Files changed:** {Resolution.files_changed joined as comma list}
+---
+
+```
+
+Commit the knowledge base update alongside the resolved session:
+```bash
+node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" commit "docs: update debug knowledge base with {slug}" --files .planning/debug/knowledge-base.md
+```
+
 Report completion and offer next steps.
 </step>