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).

server/lib/cc-discovery.js

@@ -264,8 +264,8 @@ const readOutputStyles = readSimpleMdSurface("output-styles");
  * Absolute `.md` files a plugin contributes for one surface: the conventional
  * `<root>/<subdir>` directory PLUS whatever `plugin.json` points at for that
  * key. Both matter — a manifest entry may be a single file or a directory, and
- * a plugin that declares its commands explicitly (this repo's own `ccam` lists
- * three files under `./plugins/ccam/commands/`) keeps NOTHING in `commands/`,
+ * a plugin that declares its commands explicitly (this repo's own `agenthub` lists
+ * three files under `./plugins/agenthub/commands/`) keeps NOTHING in `commands/`,
  * so reading only the conventional directory reported zero for it.
  *
  * A manifest is data from a third-party plugin, so a declared path that escapes