Add timeout to explain summary generation

[peyton-alt]

Summary

Adds a timeout to checkpoint summary generation for entire explain --generate so the command cannot hang indefinitely when the Claude CLI stalls.

What Changed

• scopes the 30 second timeout to explain --generate in explain.go
• keeps summarize.GenerateFromTranscript generic for other call sites
• preserves shorter parent deadlines and clamps longer parent deadlines to the 30 second safety timeout
• improves Claude CLI cancellation and deadline handling so context cancellation surfaces cleanly
• prints Generating checkpoint summary... while explain --generate is waiting
• adds unit coverage for default timeout, shorter parent deadline, longer parent deadline, and cancellation behavior

Why

A stalled Claude summary generation path currently leaves users waiting without a bounded failure mode. This change keeps the fix narrowly scoped to explain --generate instead of changing shared summarize behavior for other callers.

Current state before this fix:
entire explain --generate
^Cfailed to generate summary: failed to generate summary: claude CLI failed (exit -1):

Verification

• go test ./cmd/entire/cli -run 'TestGenerateCheckpointAISummary|TestExplainCmd_RejectsPositionalArgs'
• go test ./cmd/entire/cli/summarize
• mise run lint
• mise run test

[Copilot]

Pull request overview

This PR introduces a default timeout around AI summary generation to prevent the explain --generate (and other shared summarization call sites) from hanging indefinitely.

Changes:

• Add a 30s context timeout wrapper inside summarize.GenerateFromTranscript.
• Improve Claude CLI generator error handling to surface context cancellation/timeouts consistently.
• Update explain summary generation to emit a user-facing “Generating…” message and add a unit test for timeout behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
cmd/entire/cli/summarize/summarize.go	Adds a default timeout and timeout-specific error handling around generator execution.
cmd/entire/cli/summarize/generator_config_test.go	Adds a new unit test intended to validate timeout behavior.
cmd/entire/cli/summarize/claude.go	Returns context.DeadlineExceeded / context.Canceled when the context ends during Claude CLI execution.
cmd/entire/cli/explain.go	Wires through an error writer and prints progress text while generating a checkpoint summary.

[peyton-alt]

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