docs: add package ecosystem plan and error documentation workflow

Add PACKAGES.md analyzing the Lux package ecosystem gaps vs stdlib,
with prioritized implementation plans for markdown, xml, rss, frontmatter,
path, and sitemap packages. Add CLAUDE.md instructions for documenting
Lux language errors in ISSUES.md during every major task.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -71,6 +71,41 @@ nix develop --command cargo test        # All tests pass (currently 381)
 | `lux serve` | `lux s` | Static file server |
 | `lux compile` | `lux c` | Compile to binary |
 
+## Documenting Lux Language Errors
+
+When working on any major task that involves writing Lux code, **document every language error, limitation, or surprising behavior** you encounter. This log is optimized for LLM consumption so future sessions can avoid repeating mistakes.
+
+**File:** Maintain an `ISSUES.md` in the relevant project directory (e.g., `~/src/blu-site/ISSUES.md`).
+
+**Format for each entry:**
+```markdown
+## Issue N: <Short descriptive title>
+
+**Category**: Parser limitation | Type checker gap | Missing feature | Runtime error | Documentation gap
+**Severity**: High | Medium | Low
+**Status**: Open | **Fixed** (commit hash or version)
+
+<1-2 sentence description of the problem>
+
+**Reproduction:**
+```lux
+// Minimal code that triggers the issue
+```
+
+**Error message:** `<exact error text>`
+
+**Workaround:** <how to accomplish the goal despite the limitation>
+
+**Fix:** <if fixed, what was changed and where>
+```
+
+**Rules:**
+- Add new issues as you encounter them during any task
+- When a previously documented issue gets fixed, update its status to **Fixed** and note the commit/version
+- Remove entries that are no longer relevant (e.g., the feature was redesigned entirely)
+- Keep the summary table at the bottom of ISSUES.md in sync with the entries
+- Do NOT duplicate issues already documented -- check existing entries first
+
 ## Code Quality
 
 - Fix all compiler warnings before committing