feat(configstore): support local .leash.toml override file

[karimfan]

Summary

• Adds support for a project-local .leash.toml override file that gets merged on top of the global XDG config (~/.config/leash/config.toml)
• Teams can commit shared config to a repo while individual engineers keep secrets in a gitignored .leash.toml
• Local values take precedence: scalars override, maps are deep-merged

Closes #55

Changes

• internal/configstore/config.go — Merge(base, overlay Config) deep-merges two configs
• internal/configstore/path.go — LocalConfigFileName constant (.leash.toml) and GetLocalConfigPath() helper
• internal/configstore/loadsave.go — LoadWithOverlay(dir) loads global config then merges local .leash.toml on top
• internal/runner/runner.go and mount_state.go — Updated to use LoadWithOverlay(callerDir) instead of Load()

Design decisions

• Read-only local file: Save() still writes only to the global XDG config. The local .leash.toml is user-managed, avoiding accidental secret persistence in the wrong location.
• Convention over configuration: No new CLI flags needed — .leash.toml is auto-discovered in the working directory.
• Implements the "local override file" approach from the issue discussion.

Test plan

• TestMerge* — 7 tests covering empty overlay, scalar override, map merging, project-level merging, base immutability
• TestLoadWithOverlay* — 5 tests covering no local file, merge behavior, target image override, empty dir, bad TOML error
• TestGetLocalConfigPath — path helper
• All existing configstore tests pass

🤖 Generated with Claude Code

[karimfan]

@navanchauhan This implements the local override file approach you suggested in #55. Would appreciate your review!

[jaytaylor]

Thank you for your contribution, @karimfan !