fix(exec): HostMode always installs modal itself (calque#200)

[scttfrdmn]

Summary

• Found via a real calque spawn-run run against AI-Almanac's forecasts_app.py, right after calque#198's sibling-function fix correctly shipped an extra that bare-references modal: @enter failed: No module named 'modal'.
• run.go's uvPythonArgv (dry-run's own mechanism) has an explicit design decision: always include "modal" in its --with list, since a real script's body routinely references modal.Secret/modal.Volume/etc. directly even though Modal's SDK is never itself a pip_install(...) entry in the script's own .image chain.
• internal/exec/bootstrap.go's HostMode branch (used by real/fleetrun/spawn-run) had no equivalent guarantee — its uv pip install step was entirely gated behind len(PipPackages) > 0, so a run with no --pip flags (the common case; spawn-run has no --pip flag at all) installed ZERO packages, not even modal.
• HostMode now always merges "modal" into the installed package set, deduped/sorted, mirroring uvPythonArgv's exact discipline. The git-availability check (for a --pip git-URL spec) now only runs when a real --pip package was actually supplied, since modal alone needs no git.

Files touched

• internal/exec/bootstrap.go — the actual fix.
• internal/exec/bootstrap_demo_test.go — updated the now-stale "no-deps host-mode has nothing to pip-install" assertion to the corrected behavior, plus two new tests (modal merged in alongside real --pip packages; not duplicated if --pip modal is passed explicitly).
• CHANGELOG.md.

Test plan

• go build ./... && go vet ./... && gofmt -l . && go test -count=1 ./... — all pass
• golangci-lint run ./... — 0 issues
• ruff check . — all checks passed