docs: add return value comment to Capture function

[julianknutsen]

Summary

Add a comment explaining the return value semantics of the Capture function in the checkpoint package.

Related Issue

Relates to gt-i9uqg

Changes

• Added comment to Capture() in internal/checkpoint/checkpoint.go explaining:
  • What fields are populated in the returned Checkpoint
  • That git command failures are silently ignored (fields remain empty)
  • That the error return is always nil (reserved for future use)

Testing

• Unit tests pass (go test ./...)
• Build passes (go build ./...)

Checklist

• Code follows project style
• Documentation updated (if applicable)
• No breaking changes (or documented in summary)

[github-actions]

Code Coverage Report

Overall Coverage: 20.3%

Package	Coverage
cmd/gt	0.0%
internal/activity	100.0%
internal/agent	86.7%
internal/beads	27.2%
internal/boot	0.0%
internal/checkpoint	94.6%
internal/claude	0.0%
internal/cmd	9.6%
internal/config	68.2%
internal/connection	21.5%
internal/constants	0.0%
internal/crew	34.6%
internal/daemon	9.4%
internal/deacon	31.4%
internal/deps	39.6%
internal/dog	0.8%
internal/events	0.0%
internal/feed	67.3%
internal/formula	68.2%
internal/git	29.2%
internal/keepalive	60.7%
internal/lock	84.4%
internal/mail	36.0%
internal/mayor	0.0%
internal/mq	100.0%
internal/opencode	76.9%
internal/plugin	37.9%
internal/polecat	32.8%
internal/protocol	81.4%
internal/refinery	17.4%
internal/rig	41.6%
internal/runtime	82.9%
internal/session	76.6%
internal/shell	61.8%
internal/state	67.6%
internal/style	2.0%
internal/suggest	97.1%
internal/swarm	13.8%
internal/templates	31.0%
internal/tmux	34.0%
internal/townlog	46.9%
internal/tui/convoy	0.0%
internal/tui/feed	0.0%
internal/ui	13.5%
internal/util	47.6%
internal/version	0.0%
internal/web	26.4%
internal/wisp	80.7%
internal/witness	19.1%
internal/workspace	45.8%
internal/wrappers	0.0%

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