issue_prefix collision across parallel checkouts / agent worktrees

[coffeegoddd]

Problem

When multiple checkouts of the same project exist (e.g., parallel agent worktrees), they clone the same Dolt database and inherit the same issue_prefix. All checkouts then create issues in the same namespace (e.g., bd-XXXX), leading to:

1. Cross-contamination on claim/update — Agent A can claim or modify Agent B's issues because they share the prefix namespace
2. Ambiguous ownership — No way to determine which checkout created an issue
3. Search/filter pollution — bd list in one checkout shows another checkout's work

This is a blocking problem for parallel agent workflows where each checkout should own its own set of issues.

Solution

Introduce a checkout_suffix config key that appends a short unique identifier to the issue_prefix at ID generation time, giving each checkout its own issue namespace while preserving the shared project identity.

• issue_prefix = "bd", checkout_suffix = "" → IDs: bd-XXXX (unchanged, backward compatible)
• issue_prefix = "bd", checkout_suffix = "k9x" → IDs: bd-k9x-XXXX

Why a separate key instead of mutating issue_prefix?

• rename-prefix operates on issue_prefix and would need complex logic to preserve/strip suffixes
• Sync/federation compares issue_prefix across remotes — mutating it would break clone detection
• allowed_prefixes and ExtractIssuePrefixKnown already handle multi-segment prefixes
• Rollback is trivial: clear checkout_suffix to rejoin the shared namespace
• No schema migration needed — stored in the existing config key-value table

CLI interface

New --checkout-suffix flag on bd init and bd bootstrap:

• --checkout-suffix=auto — generate a random 3-char base36 suffix
• --checkout-suffix=k9x — use a specific suffix
• --checkout-suffix=none — explicitly no suffix
• Interactive prompt when no flag is provided (skipped in non-interactive/CI mode)

What changes

• ID generation uses the effective prefix (issue_prefix + optional checkout_suffix)
• ResolvePartialID adds the effective prefix to knownPrefixes for correct parsing
• MergeSlotID includes the suffix so each checkout gets its own merge slot
• Cross-checkout issues remain visible (suffix only affects creation, not reads)
• Base prefix auto-added to AllowedPrefixes for cross-checkout validation

PR

#3152