refactor: extract magic numbers to named constants

[ishaan812]

Summary

This PR improves code maintainability by replacing hardcoded numeric literals and strings with named constants. This makes the code more self-documenting and easier to maintain.

Changes

1. MaxCommitTraversalDepth constant (1000)

• Added MaxCommitTraversalDepth constant to strategy/common.go
• Replaced bare 1000 literals in 4 files:
  • strategy/common.go - IsAncestorOf() function
  • strategy/auto_commit.go - findReferencedCheckpoints() function (also standardized to use errStop sentinel)
  • rewind.go - countCommitsBetween() function
  • explain.go - computeReachableFromMain() function
• All represent the same concept: safety limit for git commit history traversal

2. Prompt truncation constants

• Added maxPromptPreviewLen = 100 in hooks_geminicli_handlers.go for log preview truncation
• Added maxDisplayPromptLen = 500 in manual_commit_condensation.go for display truncation
• Both are scoped locally to where they're used

3. RedactedPlaceholder constant

• Added RedactedPlaceholder = "REDACTED" (exported) to redact/redact.go
• Replaced bare string literal in String() function
• Makes the redaction behavior more configurable and self-documenting

Testing

• ✅ All unit tests pass (23 packages)
• ✅ Build succeeds (go build ./...)
• ✅ Code formatted with gofmt
• ✅ No new linter warnings (verified with ReadLints)

Files Changed

• cmd/entire/cli/explain.go
• cmd/entire/cli/hooks_geminicli_handlers.go
• cmd/entire/cli/rewind.go
• cmd/entire/cli/strategy/auto_commit.go
• cmd/entire/cli/strategy/common.go
• cmd/entire/cli/strategy/manual_commit_condensation.go
• redact/redact.go

Total: 7 files, +20 insertions, -11 deletions

Motivation

This addresses code quality improvements identified during codebase analysis:

• Reduces risk of inconsistent values if the limit needs to be changed
• Makes the code more self-documenting (constant names explain intent)
• Follows Go best practices for avoiding magic numbers