docs(solutions): capture the CLAUDE.md content-tiering convention

Four rounds of CLAUDE.md reduction (255 -> 305 -> 179 -> 74 lines) turned
up one consistent finding: every stale fact was a *description* of code or
config, never a rule. Two had been written by Claude days earlier.

Documents the operational test ("does this line change what Claude does on
a task where it wouldn't otherwise open the relevant file?"), the tiering
table, why gotchas are the one category that cannot move to a read-on-demand
docs/exceptions/, invariants-over-enumerations, and how to tell when a
reduction pass has hit the floor.

Also surfaces the docs/solutions frontmatter fields in CLAUDE.md's
entry-point table so the store is greppable by module, not just browsable.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -8,7 +8,7 @@ Rules, gotchas, and entry points — the things that must change what you do *be
 | Domain vocabulary — Trip, Entry, Story, Active Trip | [`CONCEPTS.md`](CONCEPTS.md) |
 | Doing something operational — posting, GPX, switching trips, local setup, deploying | [`docs/guides/`](docs/guides/) |
 | Test suite layout and conventions | [`docs/reference/testing.md`](docs/reference/testing.md) |
-| A bug or workflow trap already hit and written up | [`docs/solutions/`](docs/solutions/) — check when working in a documented area |
+| A bug or workflow trap already hit and written up | [`docs/solutions/`](docs/solutions/) — grep the `module`/`tags`/`problem_type` frontmatter; check when working in a documented area |
 | Folder map, prerequisites, the full `make` command list | [`README.md`](README.md) |
 
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