fix(agents): give every ControlHub mode the Cron tool

[bobleer]

Problem

Asked to sweep a set of channels every 30 minutes, a Cowork-mode agent answered:

我的工具列表里没有 cron 定时任务工具可用（可用的是 search/wait/浏览器等）。我会用 wait 实现 30 分钟间隔

It was telling the truth. Cron appeared in exactly one mode's default_tools — Claw — and nowhere else:

Mode	ControlHub	Cron (before)
Claw	✅	✅
Cowork	✅	❌
Team	✅	❌
DeepResearch	✅	❌
agentic / debug / multitask / plan (shared_coding_mode_tools)	✅	❌

Deferred exposure is not the cause — deferred tools are advertised to the model by name in the <deferred_tools> listing, so the agent would have seen Cron had it been in the list. It simply was not.

This also left #2212 pointing at a tool that often is not there: ControlHub's wait documentation now says to schedule repeating work with the Cron tool instead of holding the turn open, and ControlHub ships in every mode in that table. The agent got told to use a tool it did not have, and quietly fell back to chaining hour-long waits — the exact pattern that guidance exists to prevent.

Changes

• Add Cron to every mode that carries ControlHub — Cowork, Team, DeepResearch, and shared_coding_mode_tools() (agentic / debug / multitask / plan). Claw already had it.
• Add an invariant test over the built-in modes: a mode offering ControlHub must also offer Cron. This is not decoration — the first pass of this fix missed the shared coding baseline, and the test is what caught it.
• Say what to do when Cron really is absent (a custom tool manifest can still omit it) instead of leaving a chain of long wait calls as the unspoken fallback.

Scope note

This widens what the agent can propose, not what it can do unattended. CronTool::is_readonly() is false and it does not override permission_intents, so every add / update / remove / run emits a custom_tool intent and goes through the normal permission gate. Cron also stays denied in connection-scoped transient sessions, which this PR does not touch.

Testing

cargo test -p bitfun-core --lib — 1948 passed, 0 failed.

cargo fmt --check and cargo clippy unchanged from base (7 pre-existing fmt diffs, 11 pre-existing MutexGuard held across await findings, none in files this PR touches).