fix(action): post the comment before enforcing budget gates

[andreacappelletti97]

Found while building the c3x-demo showcase.

Bugs

1. Breaking a budget gate skipped the comment. The Estimate step ran with --budget (and a budget in the repo's .c3x.toml applies there too), and the Budget delta gate ran before the PR comment step. Either one failed the job first, so the pull requests that most needed a comment got none. The demo had to call the Action twice to work around it.
2. currency was ignored in comments. Only the Estimate step's log used it. Comments and the base-branch baseline were always USD.
3. The gate message said $+2781.48/mo.

Fix

• When a comment will be posted, the Estimate step passes --budget 0 and no --strict.
• Step order is now: Estimate → baseline → PR comment → Budget delta gate → Budget gate (new; applies the input or the .c3x.toml budget). strict was already enforced by the comment step after posting.
• On pushes (no comment), gating is unchanged.
• The delta gate now also runs without the input, so a budget_delta in .c3x.toml is enforced. The input is only passed when set, because passing 0 would disable the config value.
• currency is passed as --currency to the baseline and gates, and as C3X_CURRENCY to the comment step (it has no --currency flag). It's exported only when set.
• Empty-array expansion is written to be safe under set -u in bash 3.2 (macOS runners).

Tested

c3x-demo#17 ran c3xdev/c3x@fix/action-gates-after-comment on the GPU-cluster change with budget-delta: "500" and currency: EUR, in a single step:

Step	Result
Estimate	success (not gated)
Baseline	success (EUR)
PR comment	success, posted by c3x-cloud[bot]: Monthly cost increased by €2449.56 (+249.0%)
Budget delta gate	failed: delta exceeds gate — €+2449.56/mo > €+500.00/mo (the released binary; this PR fixes the sign)
Budget gate	skipped (the job had already failed)