docs: fix outdated compatibility matrix and stale references

[kacy]

summary

• compatibility.md: rewrote the full command compatibility matrix against the actual Command enum. 30+ commands added in phase 8 were still marked as ✗ (unsupported) when they're fully implemented — lists (LINDEX, LSET, LTRIM, LINSERT, LREM, LPOS), sets (SUNION, SINTER, SDIFF, SRANDMEMBER, SPOP, SMISMEMBER, SSCAN, and all *STORE variants), sorted sets (ZINCRBY, ZREVRANK, ZREVRANGE, ZRANGEBYSCORE, ZREVRANGEBYSCORE, ZCOUNT, ZPOPMIN, ZPOPMAX, ZSCAN), key lifecycle (COPY, TOUCH, RANDOMKEY, SORT, OBJECT), server (TIME, LASTSAVE, ROLE, MONITOR, CONFIG REWRITE, CLIENT ID/SETNAME/GETNAME/LIST), and transactions (WATCH, UNWATCH). also added a missing ACL section and removed the stale multi/exec entry from "commands out of scope."
• README.md: updated feature bullet lists to include phase 8 commands across strings, lists, sorted sets, sets, and key commands.
• ARCHITECTURE.md: updated command count from 114 to 135+.
• migration-from-redis.md: fixed prometheus metric names (ember_commands_duration_seconds not ember_command_duration_seconds, ember_connections_active not ember_connected_clients).
• CONTRIBUTING.md: removed stale "(wip)" from cluster module description.

what was tested

• verified every command status change against the Command enum in crates/ember-protocol/src/command/mod.rs
• verified prometheus metric names against crates/ember-server/src/metrics.rs

design considerations

compatibility.md was the worst offender — it actively undersells the project by telling users that 30+ implemented commands don't exist. this is the single most impactful doc fix for adoption since it's what people check when evaluating a Redis alternative.