fix(exec): isolate alias capture from rc-file stdout

[avihut]

Summary

• daft exec (and the -x flag on daft clone / daft init / daft checkout / daft go / daft start) became unusable for users whose .zshrc/.bashrc printed anything to stdout during rc evaluation — p10k instant prompt, plugin status banners, "welcome" echoes, fortune cookies. Their stdout output was indistinguishable from real alias output and got persisted into the cached snapshot, then re-injected into every subsequent invocation.
• Capture now writes alias/typeset -f output to dedicated temp files inside the spawned shell (paths passed via env vars). The shell's own stdout/stderr is discarded entirely, so rc-file noise can't corrupt the snapshot. A 10s deadline kills hangs from exec tmux-style rc-files. A # daft-alias-cache v2 header auto-invalidates already-poisoned caches on disk without requiring --refresh-aliases.
• The no-cache fallback is now a pristine $SHELL -c CMD instead of $SHELL -i -c CMD. Aliases won't resolve in the fallback, but the user's command runs — strictly better than today's "completely useless" failure for users whose rc files break in non-interactive contexts.
• run_exec_commands (the legacy -x path on worktree-creation flows) now reuses the same build_command, so the cache + rc-less fallback covers both surfaces.
• DAFT_EXEC_DEBUG=1 emits stderr lines for chosen execution mode, capture failures, validation rejections, and cache hits — the reporter had no logs, so the fix ships its own.

Test plan

• New unit test reproduces the bug: a polluting .bashrc previously corrupted alias_lines with POLLUTION_BEFORE_PROMPT_INIT etc.; now passes after the FD-isolated capture.
• Manual scenario tests/manual/scenarios/worktree-exec/polluted-rcfile.yml covers the end-to-end flow.
• End-to-end smoke: polluting .bashrc → fast-path resolves alias cleanly.
• End-to-end smoke: SHELL=/bin/sh → rc-less fallback, command runs.
• End-to-end smoke: planted v1 cache → header rejected, fresh capture, fresh alias used.
• End-to-end smoke: daft init -x 'alias_name' with polluting bashrc → alias resolves through cache.
• mise run fmt:check, mise run clippy, cargo test --lib core::worktree::exec all clean (47 tests pass).

Notes for users

• Existing alias caches on disk are silently invalidated on first run of this build. The next invocation triggers a fresh capture; users will see a one-time alias-capture cost (the same cost as before any cache existed). No action required.
• If shortcuts still don't resolve, run with DAFT_EXEC_DEBUG=1 and the stderr trace will show whether capture succeeded, which mode was chosen, and the constructed shell args.

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