fix(dataset push): expand ~ in the dataset path (interactive prompt resolves ~/… under CWD)

[aptracebloc]

Summary

dataset push doesn't expand a leading ~ in the dataset path, so a ~/… path resolves relative to the current directory instead of $HOME. Found while prepping a live demo of the interactive flow (#28).

Impact

Bites the interactive prompt in particular: a user types ~/tb-fixtures/tab-reg at the "Path to your dataset directory" prompt and gets:

Error: reading dataset directory "/Users/arturo/work/cli/~/tb-fixtures/tab-reg":
stat /Users/arturo/work/cli/~/tb-fixtures/tab-reg: no such file or directory

(Also affects a quoted/literal ~ passed as the positional arg, e.g. in a --no-input script.)

Why it happens

~ is expanded by the shell, not the program:

• On the command line, tracebloc dataset push ~/x → the shell rewrites ~ → $HOME before the CLI runs, so the flag form works.
• From the survey prompt the shell isn't involved — the CLI receives the literal ~/x, and push.Discover / push.DiscoverTabular call filepath.Abs(rootDir), which doesn't expand ~; it just prepends the CWD.

The fix already exists in spirit: cluster.expandPath (internal/cluster/kubeconfig.go) does ~→$HOME for --kubeconfig; the dataset path goes through nothing equivalent.

Proposed fix

• Expand ~ / ~/… → $HOME for the dataset path, mirroring cluster.expandPath.
• Apply it in the CLI layer (runDatasetPush, where a.LocalPath is resolved) so it covers both the interactive prompt and a quoted ~ arg in one place — before any Discover* call.
• (Bonus) give the interactive path prompt a survey validator that checks the directory exists and re-prompts, so a typo is caught in-flow instead of after.

Acceptance criteria

• ~/x and ~ resolve under $HOME; relative (./x) and absolute paths unchanged.
• Works for both the interactive prompt and a literal ~ positional arg.
• Unit test covering ~/x → $HOME/x, ./x, and absolute passthrough.
• (Optional) interactive path prompt validates the dir exists + re-prompts.
• make ci green.

Workaround (in place)

Use an absolute path in the interactive prompt; the demo run-sheet was updated accordingly. The command-line flag form already tolerates ~ (shell-expanded).

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