Add Claude directory flags and debug info

[abachman]

Summary

• add a --claude-dir flag so session discovery can target a specific Claude config directory
• add a --debug flag that shows discovery diagnostics in the TUI header
• cover the new discovery behavior with tests and document the new flags

Testing

• go test ./...

[github-actions]

Merging this branch will not change overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/reasonable-systems/ccsessions/cmd/ccsessions	0.00% (ø)
github.com/reasonable-systems/ccsessions/internal/claude	0.00% (ø)
github.com/reasonable-systems/ccsessions/internal/ui	0.00% (ø)

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/reasonable-systems/ccsessions/cmd/ccsessions/main.go	0.00% (ø)	0	0	0
github.com/reasonable-systems/ccsessions/internal/claude/sessions.go	0.00% (ø)	0	0	0
github.com/reasonable-systems/ccsessions/internal/ui/model.go	0.00% (ø)	0	0	0

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/reasonable-systems/ccsessions/internal/claude/sessions_test.go