Add Off collection preset (#888)

[erikdarlingdata]

Summary

• Adds a fourth value, Off, to config.apply_collection_preset. It sets enabled = 0 on every row in config.collection_schedule and returns — no frequency edits.
• The three existing presets (Aggressive, Balanced, Low-Impact) now also set enabled = 1 across the board before applying their frequency tables. A switch from Off → Balanced therefore reliably resumes collection, including for daily/on-load collectors that aren't in the preset frequency list.
• Updates the proc header comment, input validation list, PRINT summary, README blurb, and CHANGELOG.

Why this way

#888 lays out the reasoning: rather than invent a generic blackout/quiet-hours system (timezones, DST, midnight wrap-around, catch-up logic), use presets as the intensity+presence dial and let SQL Agent jobs compose time-of-day windows:

```sql
-- Agent job "Quiet hours - start" (schedule: daily 22:00)
EXECUTE config.apply_collection_preset @preset_name = N'Off';

-- Agent job "Quiet hours - end" (schedule: daily 06:00)
EXECUTE config.apply_collection_preset @preset_name = N'Balanced';
```

Zero new schema, zero new collector code, Agent owns time zones and DST, the switchover is a pair of idempotent UPDATE statements.

Test plan

Run the install script against a server, then:

```sql
-- starting state: 34/34 enabled
SELECT enabled_count = COUNT_BIG(*) FROM config.collection_schedule WHERE enabled = 1;

EXEC config.apply_collection_preset @preset_name = N'Off';
-- expected: "Applied "Off" collection preset (34 collectors disabled)"
SELECT enabled_count = COUNT_BIG(*) FROM config.collection_schedule WHERE enabled = 1;
-- expected: 0/34

EXEC config.apply_collection_preset @preset_name = N'Balanced';
-- expected: "Applied "Balanced" collection preset (29 collectors updated)"
SELECT enabled_count = COUNT_BIG(*) FROM config.collection_schedule WHERE enabled = 1;
-- expected: 34/34

EXEC config.apply_collection_preset @preset_name = N'Garbage';
-- expected: RAISERROR "Invalid preset name "Garbage". Valid presets: Off, Aggressive, Balanced, Low-Impact"
```

Verified against sql2022 — all three transitions produce the expected counts and invalid preset raises cleanly.

• Built clean (no code changes outside of SQL + docs)
• Applied to sql2022; Off → 0 enabled, Balanced → 34 enabled
• Invalid preset raises with updated error text
• Reviewer sanity check on sql2016/sql2019/sql2022 (should be identical behavior; the proc is ANSI SQL + static UPDATE)

Heads up

Applying a non-Off preset overrides any manual UPDATE config.collection_schedule SET enabled = 0 on a specific collector. That's intentional — per-collector disable/re-enable is the user's choice to make after switching presets. Documented in the proc header.

No upgrade script needed — `install/41_schedule_management.sql` uses the stub-then-`ALTER` pattern and re-runs on upgrade.

Closes #888.

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