docs: add docs/ISSUES.md tactical issue tracker + CLAUDE.md rules

Introduces `docs/ISSUES.md` as the tactical rolling list of known bugs +
small deferred features. Scope is strict: anything fitting in one-to-two
commits lives here; anything larger gets promoted to a Phase in the
roadmap. Complement, not replacement, of the strategic roadmap.

Schema per issue:
- Sequential integer ID (#1, #2, ...)
- Status (OPEN / IN-PROGRESS / DONE)
- Severity (HIGH / MEDIUM / LOW)
- Filed date, component, description, root cause, files, research,
  acceptance
- DONE items move to "Recently closed" at the bottom with commit SHA

Four seeded issues from the 2026-04-24 sky-debug session:

  #1 — Rain falls only to horizon, not to player (weather/particles)
  #2 — Lightning visual not wired (dat-baked PES triggers; research done)
  #3 — Client clock drifts from retail after ~10 min (periodic TimeSync)
  #4 — Sky horizon-glow disabled (fog-mix skipped on sky meshes)

CLAUDE.md adds an "Issue tracking" section right after roadmap
discipline with the four maintenance rules: (1) scan OPEN at session
start, (2) add/close at session end, (3) reference IDs in commit
messages, (4) promote to a Phase if an issue grows. Calls out the
strategic-vs-tactical split so future sessions know when to reach for
the roadmap vs the issues file.

README.md gets one line pointing at `docs/ISSUES.md` below the existing
roadmap link for first-class discoverability.

No code changes; doc-only.

README.md

@@ -51,6 +51,8 @@ are still stubbed or in-progress — see roadmap.
   vs retail's several kilometres
 
 See `docs/plans/2026-04-11-roadmap.md` for the ordered phase list.
+See `docs/ISSUES.md` for the rolling list of known bugs + small deferred
+features (tactical, bug-level; the roadmap is strategic, phase-level).
 
 ## Building + running