[ci] Persist VAULT_TOKEN across Woodpecker step commands

## Context
Follow-up to commit 2eca011c (bd code-e1x). That commit attached the
`terraform-state` policy to the `ci` Vault role and propagated apply-
loop failures so the pipeline actually fails when a stack fails. On
the very first push to exercise it (pipeline 361), the platform apply
step died with:

  [vault] Starting apply...
  state-sync: ERROR — no Vault token and no age key at ~/.config/sops/age/keys.txt
  [vault] FAILED (exit 1)

Root cause: in Woodpecker's `commands:` list, each `- |` item runs in
a fresh shell. The dedicated "Vault auth" command was doing
`export VAULT_TOKEN=...`, but that export was lost by the time the
apply command ran. Tier-0 stacks depended on Vault Transit (via
`scripts/state-sync`), and Tier-1 stacks depend on
`vault read database/static-creds/pg-terraform-state` via `scripts/tg`
— both silently fell through to their "no Vault" error path.

This bug was latent before 2eca011c because the old apply loop
swallowed per-stack exit codes. Now that we surface them, the pipeline
fails honestly — but fails on every run. Fixing the missing token
propagation is the last mile.

## This change
- Pin `VAULT_ADDR` at the step's `environment:` level so every command
  inherits it without an explicit export.
- In the Vault auth command, assert the auth succeeded (non-empty,
  non-"null" token) then write the token to `~/.vault-token` with
  `umask 077`. `vault`, `scripts/tg`, and `scripts/state-sync` all
  fall through to `~/.vault-token` when `VAULT_TOKEN` env is unset.

## What is NOT in this change
- A broader refactor to fold the multi-step chain into a single
  `- |` script — preserving the existing granular structure keeps
  individual step logs grep-friendly and failures localised.
- Restoring the VAULT_TOKEN export too — redundant once ~/.vault-token
  is written, and would need duplicating into each command anyway.

## Test Plan
### Automated
N/A (pure YAML change). Will be verified by the very next CI run —
the push creating this commit.

### Manual Verification
Watch `ci.viktorbarzin.me/repos/1/pipelines` for the pipeline whose
commit matches this one. Expected:
- `default` workflow exercises the auth + apply steps.
- Platform apply for `vault` stack runs state-sync decrypt → detects
  no drift (I applied locally already) → OK.
- Tier-1 stacks (if any in the diff): `vault read database/static-
  creds/pg-terraform-state` returns creds → apply runs.
- No "state-sync: ERROR" or "Cannot read PG credentials" errors.
- `default` workflow state: success.
- Overall pipeline status: still failure because `build-cli` is
  independently broken (bd code-12b); that's cosmetic.

Refs: bd code-e1x

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.woodpecker/default.yml

@@ -37,6 +37,12 @@ steps:
     environment:
       SLACK_WEBHOOK:
         from_secret: slack_webhook
+      # Each `- |` command runs in a fresh shell, so we can't rely on an
+      # `export VAULT_ADDR=...` in the auth command persisting — pin it at
+      # step level. VAULT_TOKEN is still per-command; we persist it to
+      # ~/.vault-token (auto-read by `vault` CLI) so downstream commands
+      # don't need explicit token propagation.
+      VAULT_ADDR: http://vault-active.vault.svc.cluster.local:8200
     commands:
       # ── Skip CI commits ──
       - |
@@ -55,9 +61,17 @@ steps:
       # ── Vault auth ──
       - |
         SA_TOKEN=$(cat /var/run/secrets/kubernetes.io/serviceaccount/token)
-        export VAULT_ADDR=http://vault-active.vault.svc.cluster.local:8200
-        export VAULT_TOKEN=$(curl -s -X POST "$VAULT_ADDR/v1/auth/kubernetes/login" \
+        VAULT_TOKEN=$(curl -s -X POST "$VAULT_ADDR/v1/auth/kubernetes/login" \
           -d "{\"role\":\"ci\",\"jwt\":\"$SA_TOKEN\"}" | jq -r .auth.client_token)
+        if [ -z "$VAULT_TOKEN" ] || [ "$VAULT_TOKEN" = "null" ]; then
+          echo "ERROR: Vault K8s auth failed (role=ci, ns=woodpecker)" >&2
+          exit 1
+        fi
+        # Persist for downstream `- |` blocks (each runs in a fresh shell,
+        # so exporting VAULT_TOKEN wouldn't help). `vault`, `scripts/tg`,
+        # and `scripts/state-sync` all fall through to ~/.vault-token when
+        # the env var is unset.
+        umask 077; printf '%s' "$VAULT_TOKEN" > "$HOME/.vault-token"
 
       # ── Detect changed stacks ──
       - |