Clarify and honor saved context page continuation - #8318

Problem

A bounded saved-context opening can reach EOF while the initial read still has a required remainder. Its page instruction points only to the ordinary cursor within that selection. A caller can miss the remainder and reconstruct the pre-work read with broader manual selections. The same entry point rejects an explicit --page-bytes before reaching its window validator.

Change

• Point the opening's existing page instruction to navigation.remaining_read_command and its cursor before work.
• Let explicit page windows reach the existing validator for attached initial and selected reads.
• Exercise an ordinary complete page, a staged opening and remainder, tight and explicit windows, invalid limits, and recovery of the same captured decision.

The saved-context CLI transport owns the instruction. The same captured Goal, Agent, Turn and digest bind the opening and remainder.

Validation

• Ran the benchmark-profile CLI on the same immutable input at base and head. Both delivered the same 69 selected addresses and original receipt through the opening and remainder. The candidate opening fit within the default 12,000-byte serialized response limit.
• Exact-head local quick premerge, public boundary, Ruff and bilingual diff checks passed.
• Saved context and first-read suites produced 137 passing cases and 14 skips. Two warm native checkpoint tests expect delta and receive full delivery on both this head and the exact main base. All seven focused page-window cases pass, including three restored window cases.
• Live model follow-through and whole-turn savings remain to be measured.