fix(install): surface valid-agent hint in unknown-agent error

[sunilkumarvalmiki]

Improve the unknown-agent error message to surface valid-agent discovery hints, closing the asymmetry with the unknown-tool error path.

What this PR does

• Replaces the terse unknown-agent message with a multi-line hint listing example slugs, the full roster discovery path, and the total agent count (273 agents across 18 divisions).
• Adds the same --list agents hint to the unknown-agent-in-agents-file error path.
• Extends test-agent-selection.sh with a new case asserting the error surfaces a valid-option hint.
• Fixes a pre-existing quoting bug in test-agent-selection.sh ($INSTALLER was unquoted, causing the test to fail on repo paths containing spaces).

Checklist

• Two file changes only.
• Regression test added (case 4).
• Tested locally: test-agent-selection.sh passes all cases; check-tools.sh, check-divisions.sh, and check-runbooks.sh pass.
• Proofread: error message mirrors the established unknown-tool pattern.

Evidence

• unknown-agent selection test: PASS
• agents-file selection test: PASS
• valid display-name selection test: PASS
• post-improvement discovery-hint test: PASS
• check-tools.sh: PASSED (16 tools consistent)
• check-divisions.sh: PASSED (18 divisions consistent)
• check-runbooks.sh: PASSED (4 runbooks, 64 agent slug references)
• CRLF check: no CRLF detected