fix(limits): clamp CODEMAN_MAX_SESSIONS to 500 and document it

Review follow-up. Other bounds assume a small session count: an owner's
tab layout holds at most MAX_TAB_REFS (512) refs and PUT
/api/session-order accepts at most 500 ids, so with
CODEMAN_MAX_SESSIONS=600 the 513th session creation answered HTTP 500
instead of the capacity message. The value is now clamped to a named
MAX_CONCURRENT_SESSIONS_CEILING of 500; any digit string above it
(including one past the safe-integer range) resolves to 500 instead of
falling back to 50. The resolver reads process.env directly, the unused
env parameter is gone.

Docs: a CODEMAN_MAX_SESSIONS row in the Settings Reference environment
table, the env name next to the cap in the cron guide and the
architecture invariants, and both copies of the skill's endpoints
reference now give the per-user cap as half the global cap (25 by
default). The changeset says "1 to 500". The test drops the
`as unknown as string` casts and adds '1000', '501' and an
out-of-range value, each refusing at 500.

Co-Authored-By: Christian Haberl <116439789+christianhaberl@users.noreply.github.com>
Co-Authored-By: Claude <noreply@anthropic.com>

.changeset/max-sessions-env.md

@@ -2,4 +2,4 @@
 'aicodeman': minor
 ---
 
-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.
+The cap of 50 concurrent sessions can now be set with `CODEMAN_MAX_SESSIONS` (1 to 500, read at startup; a larger value is clamped to 500), 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.