Simplify README: assume linux/amd64, remove go install, fix nested code blocks

[Copilot]

Simplifies README by assuming linux/amd64 platform and fixes markdown rendering issues from improperly nested code blocks.

Changes

• Installation: Removed go install section
• Bootstrap scripts: Hardcode linux_amd64 instead of detecting OS/architecture in jira-cli and slack-cli examples
• Best practices: Removed platform detection example
• Code snippets: Converted nested fenced code blocks to indented blocks within heredoc examples to fix markdown rendering

Before/After

Before (broken nested code blocks):

cat > task.md << 'EOF'
## Example
```bash
command here

EOF

**After** (properly rendered):
```bash
cat > task.md << 'EOF'
## Example

    command here

EOF

Affected examples: implement-jira-story, triage-jira-bug, respond-to-jira-comment, notify-build-status, notify-deployment

Original prompt

Please simplify the README by downloading jira/slack cli directly without bothering to determine the correct arch (assume linux/x86). Remove the install using "go install". It seems like some of the code snippets have gone wrong.

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