ci: make CI fork-friendly when DEPLOY_KEY secret is absent

The fork's CI has never run (`gh api repos/quangdang46/jcode/actions/runs`
returns total_count=0 for both pull_request and push events) because
upstream's workflows assume a `DEPLOY_KEY` SSH secret that the fork
does not have.

Two changes:

1. Gate every `webfactory/ssh-agent@v0.9.0` step on
   `if: ${{ secrets.DEPLOY_KEY != '' }}` so the SSH agent setup is
   simply skipped when the secret is absent. This is safe here because
   none of this fork's Cargo git dependencies use SSH URLs:
   - `agentgrep` -> https://github.com/1jehuang/agentgrep.git
   - `mermaid-rs-renderer` -> https://github.com/1jehuang/mermaid-rs-renderer.git
   So the `actions/checkout@v4` step's empty `ssh-key` falls back to
   the GITHUB_TOKEN HTTPS path automatically. Applies to 5 jobs in
   ci.yml + 1 step in release.yml.

2. Add `workflow_dispatch:` to ci.yml so maintainers can trigger CI
   manually from the Actions tab while debugging without having to
   force-push a no-op commit.

Note for the fork owner: GitHub disables Actions on a freshly-forked
repo until you visit the Actions tab once and click 'I understand my
workflows, go ahead and enable them'. After this PR is merged AND the
fork's Actions are enabled, future PRs will automatically run CI.

Refs the absence of CI runs blocking PRs quangdang46#174-quangdang46#195.

Author: quangdang46

.github/workflows/ci.yml

@@ -5,6 +5,7 @@ on:
     branches: [main, master]
   pull_request:
     branches: [main, master]
+  workflow_dispatch:
 
 concurrency:
   group: ci-${{ github.workflow }}-${{ github.ref }}
@@ -26,6 +27,7 @@ jobs:
           submodules: recursive
 
       - name: Configure SSH for cargo git dependencies
+        if: ${{ secrets.DEPLOY_KEY != '' }}
         uses: webfactory/ssh-agent@v0.9.0
         with:
           ssh-private-key: ${{ secrets.DEPLOY_KEY }}
@@ -79,6 +81,7 @@ jobs:
           submodules: recursive
 
       - name: Configure SSH for cargo git dependencies
+        if: ${{ secrets.DEPLOY_KEY != '' }}
         uses: webfactory/ssh-agent@v0.9.0
         with:
           ssh-private-key: ${{ secrets.DEPLOY_KEY }}
@@ -120,6 +123,7 @@ jobs:
           submodules: recursive
 
       - name: Configure SSH for cargo git dependencies
+        if: ${{ secrets.DEPLOY_KEY != '' }}
         uses: webfactory/ssh-agent@v0.9.0
         with:
           ssh-private-key: ${{ secrets.DEPLOY_KEY }}
@@ -209,6 +213,7 @@ jobs:
           submodules: recursive
 
       - name: Configure SSH for cargo git dependencies
+        if: ${{ secrets.DEPLOY_KEY != '' }}
         uses: webfactory/ssh-agent@v0.9.0
         with:
           ssh-private-key: ${{ secrets.DEPLOY_KEY }}
@@ -380,6 +385,7 @@ jobs:
           submodules: recursive
 
       - name: Configure SSH for cargo git dependencies
+        if: ${{ secrets.DEPLOY_KEY != '' }}
         uses: webfactory/ssh-agent@v0.9.0
         with:
           ssh-private-key: ${{ secrets.DEPLOY_KEY }}

.github/workflows/release.yml

@@ -51,6 +51,7 @@ jobs:
           fetch-depth: 0
 
       - name: Configure SSH for cargo git dependencies
+        if: ${{ secrets.DEPLOY_KEY != '' }}
         uses: webfactory/ssh-agent@v0.9.0
         with:
           ssh-private-key: ${{ secrets.DEPLOY_KEY }}