feat(runtime): L1 resumable executor (execActions) + readable tail + fragment index

[joelgwebber]

The runtime primitive we drove JetBlue's checkout through all along, graduated
from spike. Additive; touches only the SightkickGlobal host/console API.

execActions(actions, args, options) -> ActionStatus

Runs an arbitrary Step[] with the same atomic step machinery as runTool,
but swaps the outcome envelope: instead of throwing / hanging at the first action
that doesn't cleanly resolve, it returns a structured status —
{ completedThrough, total, done, interrupt?{at, action, reason, observed}, remaining, remainingView }.

• remaining is the untried tail beginning with the interrupted action — a
  transparent continuation the caller resubmits to resume, either blind (opaque
  continuation) or after editing a step (escape hatch).
• remainingView is a readable {op, target} projection of that tail — keeps the
  predicate discriminators ([label*="…"]) instead of dumping raw locator JSON.

fragments() / projectFragments(ir)

Projects the loaded IR's tool steps into a flat, pluckable index —
{ id, tool, index, op, label, uses } — so an agent composing an execActions
list finds a step by the params it fills (uses includes firstName) instead of
scanning ir.tools[].steps and string-matching interpolation values.

Scope / non-scope

Both are additive methods on window.__sightkick (the host/console API we
drive via browser eval and the sightkick-debug flow) — not WebMCP tools:
boot.ts still registers only ir.tools on document.modelContext. This is the
primitive a future "fallback" exec tool would wrap; nothing here registers or
forecloses that.

Validated end-to-end driving JetBlue cart → checkout → contact; runtime suite
green (execActions interrupt/resume, readable-projection, and fragment-index
tests).