feat(limits): make the global session cap configurable with CODEMAN_MAX_SESSIONS

The cap of 50 concurrent sessions was a hard-coded constant, so a machine
with the memory for more (or a small box that wants fewer) had to patch the
source. CODEMAN_MAX_SESSIONS now sets it at startup. The default stays 50,
and anything that is not a positive integer (empty, 0, negative, fractional,
1e3, out of safe-integer range) falls back to it, so a typo never locks
session creation.

Every site keeps reading MAX_CONCURRENT_SESSIONS, so the session routes,
cron launches, the scheduled-run loop and the multi-user per-user cap (half
the global cap unless CODEMAN_MAX_SESSIONS_PER_USER is set) all follow the
variable. The test goes through sessionCapacityState and
sessionCapacityMessage, the check every session-creating route uses.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/cron-guide.md

@@ -261,8 +261,8 @@ Sequence in `CronService.launch()`:
 4. **Auto-close previous session** (recurring schedules, unless
    `autoClosePreviousSession: false`): any still-open session created by this
    job's previous runs is closed via the normal session-cleanup path.
-5. The global session cap is checked (`MAX_CONCURRENT_SESSIONS = 50`). At cap →
-   **`failed`**.
+5. The global session cap is checked (`MAX_CONCURRENT_SESSIONS`, 50 unless
+   `CODEMAN_MAX_SESSIONS` sets it). At cap → **`failed`**.
 6. A `Session` is created **with `useMux: true`** (so it runs inside tmux),
    registered, listeners attached, and started via `startInteractive()`
    (`startShell()` for `shell` mode). Model/claudeMode come from global config.