docs(readme): installer links at top + fenced code blocks for copy button (#266, #268)

[MScottAdams]

Summary

• Move installer asset links adjacent to the Getting Started section so platform downloads are visible near install instructions
• Wrap install/run commands in fenced code blocks with language tags so GitHub renders copy buttons
• Mark spec tasks t2.10.1 and t2.10.2 as completed and add CHANGELOG entries under [Unreleased]

Related Issues

Closes #266
Closes #268

References: t2.10.1, t2.10.2

Checklist

• /deft:change <name> -- N/A: task was explicitly constrained to README.md, SPECIFICATION.md, and CHANGELOG.md only
• CHANGELOG.md -- added entries under [Unreleased]
• ROADMAP.md -- N/A (release-time update convention)
• Tests pass locally -- uv run pytest tests/content/ -x -q passes; task check and full pytest hit a pre-existing Windows pytest teardown error (WinError 448) after test execution completes

[greptile-apps]

Greptile Summary

This PR adds installer asset download links adjacent to the Getting Started section in README.md and wraps all install/run commands in fenced code blocks with bash language tags so GitHub renders copy buttons. It also marks spec tasks t2.10.1 and t2.10.2 as completed and adds the corresponding CHANGELOG.md entries. A follow-up commit (d868d5b) resolved the previously flagged duplicate ### Changed subsection in [Unreleased] — the changelog structure is now correct.

Confidence Score: 5/5

Safe to merge — documentation-only changes with no logic, no regressions, and the previously flagged duplicate changelog section resolved.

All three changed files are documentation. The README additions are correct and complete (download callout, Quick Download links, fenced code blocks on every command). The SPECIFICATION tasks are correctly marked completed. The duplicate ### Changed issue flagged in the prior review thread was addressed by the follow-up commit, leaving a clean single-section structure. No P0/P1 findings remain.

No files require special attention.

Important Files Changed

Filename	Overview
README.md	Adds Quick Download callout (lines 27–30) and a TL;DR-area "Get Deft" blockquote (line 21); wraps macOS, Linux, build-from-source, and git-clone commands in ```bash fenced blocks — all correct and complete.
CHANGELOG.md	The previously flagged duplicate ### Changed subsection was merged by the fix commit; the [Unreleased] block now has one ### Fixed and one ### Changed section. A cosmetic double blank line remains between ## [Unreleased] and ### Fixed (lines 9–10).
SPECIFICATION.md	Tasks t2.10.1 and t2.10.2 correctly flipped from pending to [completed]; acceptance criteria match the README changes.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["User lands on README.md"] --> B{"Scrolls past TL;DR?"}
    B -- No --> C["Sees ⬇️ Get Deft blockquote\n(line 21)\nGitHub Releases link"]
    B -- Yes --> D["Reaches 🚀 Getting Started\n(line 23)"]
    C --> D
    D --> E["⬇️ Quick Download callout\nDirect binary links\nWindows / macOS / Linux\n(lines 27–30)"]
    E --> F["Platform install steps\nwith fenced bash blocks\n(copy-button enabled)"]
    F --> G["Windows — GUI only\n(no fenced block needed)"]
    F --> H["macOS — chmod+run\n+ xattr quarantine removal\n```bash blocks```"]
    F --> I["Linux — chmod+run\n```bash block```"]
    F --> J["Build from source / git clone\n```bash blocks```"]

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_{Reviews (3): Last reviewed commit: "fix(changelog): merge duplicate Unreleas..." | Re-trigger Greptile}

[MScottAdams]

rebase-only onto master (post-#276 merge) -- no logic changes