claude-obsidian: meter and cap usage; cut per-session cost ~5.6x

The user asked for usage to be monitored closely, and measuring it turned
up that most of the spend was avoidable.

Metering: every model call is appended to usage.jsonl with BILLED tokens
(modelUsage -- the visible `usage` turn undercounted by half) and the
CLI's list-price cost. `usage` reports 5h/24h/7d. Each call's cost is
estimated first and refused if it would cross the per-run or rolling 24h
budget, so budgets cannot be overshot. Removed --max-budget-usd: it
aborts after spending and discards the result ($0.26 lost in one call).

Cost, measured on one session ($0.057 -> $0.010, same summary content):
- alwaysThinkingEnabled=false: extended thinking was ~85% of output.
- DISABLE_PROMPT_CACHING=1: one-shot prompts were being written to the
  1h cache at 2x input price.
- CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC=1: a hidden side request
  re-read the whole prompt, doubling billed input (9,638 vs 4,663).

Summaries are cached until their note is written, so a vault failure
never re-pays; --prefetch summarizes while the vault is unreachable.
Offline tests (fake claude binary): budget refusal pre-flight, cache
reuse with zero calls, map-reduce, env/argv levers, billed-token ledger.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KF322NzozeaFjpgWvh4F4g

claude-obsidian/config.env.example

@@ -21,3 +21,5 @@ VAULT_FOLDER=Claude
 SETTLE_MINUTES=30     # leave sessions alone until they have been idle this long
 JOBS=1
 MAX_PER_RUN=10        # summaries share your plan's usage limits; the timer drips a backlog
+BUDGET_USD_PER_RUN=0.50   # list-price USD, refused pre-flight; see `claude-obsidian-sync usage`
+BUDGET_USD_PER_DAY=2.00