feat: copy repository files into new worktrees

[abiswas97]

Summary

• add top-level .sentei.yaml worktree_files rules with explicit container-relative sources, worktree-relative destinations, and opt-in overwrite
• copy files during the shared creator Setup phase with traversal and symlink containment checks, nested parent creation, atomic writes, permission preservation, and visible failures
• keep env_files independent while forwarding automatic rules through both CLI and TUI creation
• document configuration semantics and cover config, copy safety, orchestration, UI, CLI, and realistic bare-container behavior

Test plan

• go test -race -count=1 ./...
• go test -tags=e2e -count=1 ./...
• go vet ./...
• golangci-lint run ./...
• go build ./...
• goreleaser check
• goreleaser build --snapshot --clean
• branch-binary smoke test against a fresh .bare Sentei container, verifying nested .codex/config.toml, copied content, and mode 0600

[codecov]

Codecov Report

❌ Patch coverage is 84.58333% with 37 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/worktreefile/copy.go	83.46%	12 Missing and 9 partials ⚠️
internal/creator/env_files.go	68.00%	4 Missing and 4 partials ⚠️
internal/creator/prepared.go	50.00%	2 Missing and 2 partials ⚠️
internal/creator/worktree_files.go	84.61%	1 Missing and 1 partial ⚠️
internal/worktreefile/rule.go	91.30%	1 Missing and 1 partial ⚠️

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