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>

.changeset/max-sessions-env.md

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+The cap of 50 concurrent sessions can now be set with `CODEMAN_MAX_SESSIONS` (a positive integer, read at startup), for a machine with the memory for more sessions or a small one that should run fewer. Without it nothing changes. A value that is not a positive integer falls back to 50, so a typo never blocks session creation. In multi-user mode the per-user cap still defaults to half the global cap, so it follows the new value unless `CODEMAN_MAX_SESSIONS_PER_USER` is set.