refactor!: rebrand CCAM to AgentHub

Clean break, no compatibility alias. Renames the product name, the CLI
binary (`ccam` -> `agenthub`), the env var prefix (`CCAM_` ->
`AGENTHUB_`), the Prometheus metric namespace (`ccam_*` ->
`agenthub_*`), the tmux session prefix (`ccam-lane-<id>` ->
`agenthub-lane-<id>`), the per-repo runtime profile directory
(`.ccam/profile` -> `.agenthub/profile`), the lanes state root
(`~/.claude/ccam-lanes` -> `~/.claude/agenthub-lanes`), the bundled
plugins (`plugins/ccam*` -> `plugins/agenthub*`) and their slash
commands (`/ccam-doctor` -> `/agenthub-doctor`, `/ccam:*` ->
`/agenthub:*`).

Because there is no alias, an existing install has to be updated by
hand: rename its `.ccam/profile` directory and any `CCAM_*` variables,
and reinstall the plugin under its new name.

Two other name families are deliberately left alone here, since they are
not just strings: the `Claude-Code-Monitor` repository/marketplace name,
and the `agent-dashboard` npm package plus its `~/.claude/agent-dashboard`
runtime directory, which holds the live database and needs a migration
rather than a rename.

Verified: server 1406/1406, client 459/459, `grep -i ccam` clean across
tracked files, `openapi.yaml` regenerated (95 paths, unchanged count).

plugins/agenthub-cost-guard/commands/budget.md

@@ -0,0 +1,24 @@
+---
+description: Show current Claude Code spend versus a budget number
+argument-hint: "[monthly-$]"
+---
+
+Show current Claude Code spend against the budget **$ARGUMENTS** (a monthly dollar
+figure, e.g. `50`). If `$ARGUMENTS` is empty, just report spend-to-date and ask for a budget.
+
+1. Fetch current spend:
+   ```
+   curl -s http://localhost:4820/api/pricing/cost
+   ```
+   Read `total_cost` (spend-to-date) and the per-model `breakdown`.
+
+2. Print a concise budget status:
+   - **Spend to date**: `total_cost` as USD to 4 decimals.
+   - **Budget**: `$ARGUMENTS`/month.
+   - **Consumed**: `total_cost / budget × 100`% — with a ▲/▼ vs. the linear pace expected for today's day-of-month (`budget × day_of_month / days_in_month`).
+   - **Verdict**: under budget / on pace / over budget.
+   - **Top 3 models by cost** from the breakdown (model, cost, % of total).
+
+If the dashboard is unreachable (curl fails / empty), tell the user to start it with
+`npm start` from the repo root. Keep the output to a few lines plus one small table.
+Currency as USD to 4 decimal places. Read-only — do not modify anything.