feat(coding-agents): spread page refreshes with hashed cron fields (H)

[nicoloboschi]

Alternative to #4176, addressing the same problem with a different interface.

The problem (same as #4176)

One pageTriggerCron is shared by every page in every bank running this plugin. A literal
"0 3 * * *" therefore does not schedule a refresh at 03:00 — it schedules all of them at
03:00, ~5 page syntheses per bank, on the worker pool that also serves retain. A session ingesting
at 03:0x queues behind the pile. Moving the hour moves the pile.

The interface

#4176 adds a pageTriggerType member, "daily-staggered". That spells the period in the type name,
so the next ask (hourly, or nightly-only) is another enum member, another branch, another docs row —
and "daily-staggered" cannot express "spread within 03:00" at all.

Spreading is a property of the schedule, so put it in the expression. A cron field written H is
replaced, per page, by a value hashed from bank + page name:

pageTriggerCron	what each page gets
"H H * * *"	once a day, at its own minute and hour (= #4176)
"H * * * *"	once an hour, at its own minute
"H 3 * * *"	daily at 03:MM — spread inside the hour you chose
"H H(0-5) * * *"	daily, spread across the night only
"0 3 * * *"	unchanged — no H, no hashing

H is Jenkins' syntax for exactly
this problem, borrowed rather than invented so it is recognisable. No new config field, no new enum
member.

Implementation notes

• H never leaves the package. expandCronHash resolves it to an ordinary 5-field expression, so
  the API parses plain cron and the control plane shows an editable schedule.
• Resolution happens at page creation (seedPages, captureInitiative) rather than in
  buildPageTrigger, because that is the only point where the page's identity exists — the trigger
  is built once per session for all of them.
• The field index is hashed alongside the seed, so H H * * * is worth 1440 slots rather than 60
  correlated ones (a test asserts all 24 hours are reached across 200 banks).
• A malformed H is refused at config resolution with a fallback to auto-refresh, matching how a
  missing pageTriggerCron is already handled. expandCronHash deliberately leaves an expression it
  cannot read alone rather than inventing a time nobody asked for.
• Hashing distributes approximately; two pages can still collide, just not all of them.

Not included

#4176's second change — reconciliation patching only tags_match so a hand-edited schedule survives
— is an independent fix and is left to that PR.

Validation

892 tests pass (67 files), package build, tsc --noEmit, and ./scripts/hooks/lint.sh all green.
skill/SKILL.md, the docs page and the docs-skill reference were regenerated from the README after
lint, per CLAUDE.md.