Removes the `handoff` feature flag entirely and un-gates context
compaction so auto-compaction and the `/compact` command are available
to everyone. The `HandoffFeatureFlag` definition is deleted, and every
flag check is removed: auto-compaction always runs in the agent turn
loop (still governed by the `agent.auto_compact` setting and the model's
context window size), the `/compact` slash command is always registered
and routed to manual compaction, the token-limit callout always defers
to auto-compaction when the window is large enough, and the Auto Compact
settings always appear in the settings UI.

Merging this is the switch that ships the feature; until then it stays
stacked behind the telemetry and retry-fix PRs.

Release Notes:

- Added auto-compaction and /compact to Zed Agent

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Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>