feat(installer): add Linux installer script for release binaries

[mchurichi]

What

• Added scripts/get-peek.sh, a POSIX sh Linux installer/uninstaller for release binaries.
• Added install flows for latest/specific version, user/system install targets, and uninstall with optional --purge and --force.
• Added architecture/downloader/checksum handling in the script (amd64/arm64, curl/wget, SHA-256 verification).
• Updated consumer docs in README.md with install/uninstall usage examples.
• Updated technical docs in docs/README.md with installer behavior and GoReleaser naming/checksum coupling.
• Updated AGENTS.md to document new commands, structure entry, and installer compatibility rule.

Why

Installing peek from GitHub releases on Linux was manual and error-prone.
This adds a single script for consistent install/uninstall behavior and documents the expected release artifact contract so future release changes do not break installation.

Related Issue

• N/A

Testing Done

• Manual script-path validation and docs update review on branch.
• scripts/get-peek.sh includes non-interactive safety for uninstall --purge (requires --force when no TTY).

[github-actions]

Release Label Suggestion

Suggested label: release:minor
If merged now, estimated next tag: v0.5.0
Why: Conventional prefix in title/header: feat (source: PR title)

You can change this by updating PR labels before merge.

Allowed labels (exactly one required):

• release:patch
• release:minor
• release:major
• skip-release

[mchurichi]

@codex review this PR

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

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