Fix explain showing "(no prompt)" for multi-session checkpoints

[khaong]

Summary

• ReadLatestSessionPromptFromCommittedTree now iterates backwards through sessions when the latest has no prompt.txt, falling back to earlier sessions that do have a prompt
• Fixes entire explain showing "(no prompt)" for multi-session checkpoints where a contaminated/empty session was condensed as the highest-indexed session (see bug add .entire/logs to .entire/.gitignore, small refactor #3 in docs/plans/2026-03-05-explain-bugs.md)

Test plan

• Added test: falls back to earlier session when latest has no prompt
• Added test: falls back through multiple empty sessions to find prompt
• Added test: returns empty when no session has a prompt
• Added test: empty prompt.txt triggers fallback
• All existing TestReadLatestSessionPromptFromCommittedTree subtests pass
• Full strategy test suite passes

🤖 Generated with Claude Code

[cursor]

PR Summary

Low Risk
Low risk: small, localized change to prompt lookup logic plus additional tests; behavior only changes when the latest session lacks a usable prompt.txt.

Overview
Fixes prompt resolution for committed multi-session checkpoints by updating ReadLatestSessionPromptFromCommittedTree to scan backward from the latest session until it finds a non-empty prompt, instead of returning empty when the latest session is missing/empty.

Adds unit tests covering fallback across missing prompt.txt, empty prompt.txt, multiple empty sessions, and the all-missing case.

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[Copilot]

Pull request overview

This PR fixes entire explain showing “(no prompt)” for committed checkpoints that contain multiple sessions where the latest/highest-indexed session doesn’t have a usable prompt.txt. It updates the prompt lookup logic to search backward through sessions until it finds a non-empty prompt, which better matches how condensed checkpoints can be laid out on the metadata branch.

Changes:

• Update ReadLatestSessionPromptFromCommittedTree to iterate backward across sessions and return the first non-empty extracted prompt.
• Add unit tests covering fallback behavior across missing/empty prompt.txt scenarios, including multiple empty sessions and the “no prompt anywhere” case.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
cmd/entire/cli/strategy/common.go	Implements backward fallback across session directories when the latest session has no usable prompt.
cmd/entire/cli/strategy/common_test.go	Adds targeted tests to validate fallback behavior for missing/empty prompts across sessions.