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/README.md

@@ -36,6 +36,8 @@ claude.ai export .zip ────────┼─ condense → redact → sum
 claude-obsidian-sync sync                          # what the timer runs
 claude-obsidian-sync sync --session <session-id>   # one session, now, ignoring the settle window
 claude-obsidian-sync import-claude-ai ~/Downloads/data-2026-09-13.zip
+claude-obsidian-sync sync --prefetch --limit 3     # summarize into the local cache, vault untouched
+claude-obsidian-sync usage                         # calls, billed tokens, spend: last 5h / 24h / 7d
 ```
 
 Claude reads the vault through the plugin's MCP server:
@@ -53,6 +55,22 @@ claude mcp add --scope user --transport http obsidian http://<host>:27123/mcp/ \
   subscription has no key of its own, so a background job cannot use its `claude-*` models.
 - **`--no-session-persistence` is load-bearing.** Without it every summary writes a new
   transcript into `~/.claude/projects`, and the next run summarizes its own summarizing.
+- **Usage is metered and capped.** Every model call is appended to
+  `~/.local/state/claude-obsidian/usage.jsonl` with *billed* tokens (the CLI's `modelUsage`, not
+  the visible turn) and `total_cost_usd` — API list-price equivalent, a faithful proxy for how much
+  was consumed; the plan's own meter is only visible in Claude Code's `/usage`. Before each call
+  its cost is **estimated** (+20% margin) and the call is refused if it would cross
+  `BUDGET_USD_PER_RUN` or the rolling `BUDGET_USD_PER_DAY` — so budgets are never overshot. Not
+  `claude -p --max-budget-usd`: that aborts *after* the tokens are spent and discards the result
+  (measured: $0.26 for nothing). Every run ends with a one-line usage report.
+- **Three cost levers, measured on one session — $0.057 → $0.010, same summary content:**
+  `alwaysThinkingEnabled=false` (extended thinking was ~85% of output tokens),
+  `DISABLE_PROMPT_CACHING=1` (the CLI writes each prompt to the 1-hour cache at 2× input price, and
+  a one-shot call never reads it back), and `CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC=1` (otherwise a
+  hidden side request re-reads the entire prompt, doubling billed input). A whole ~110-session
+  backlog is about $2 at list price with all three.
+- **Summaries are cached until their note is written**, so a vault outage never pays for the same
+  summary twice, and `--prefetch` can do the summarizing while the vault is unreachable.
 - **It shares your plan's usage limits.** A first backfill of ~100 sessions in parallel hit a
   session limit within minutes — which also limits *interactive* use. So: one job at a time,
   `MAX_PER_RUN` sessions per run (newest first), and the run **stops at the first usage-limit