[FEATURE] Replace autoCompact boolean with off/auto/ask modes

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

autoCompactEnabled is currently a boolean, which gives very limited control over when compaction happens. The only way to get anything resembling an "ask before compacting" behavior today is to manually toggle autoCompactEnabled on and off mid-session while watching the token count on screen — tedious, error-prone, and a strong signal that a two-state setting isn't enough to express what developers actually want.

Proposed Solution

Replace the boolean with a three-state setting:

"autoCompact": "off" | "auto" | "ask"

• off — never compact automatically (today's false)
• auto — current behavior (today's true)
• ask — prompt before compacting instead of forcing a binary always-on/always-off choice

This mode was previously proposed in #63232 as "autoCompact": "prompt", but that issue was auto-closed as a duplicate of #47145 (a bug report about unconsented data loss, not a feature spec) with no maintainer follow-up. Re-raising it here as a standalone, scoped request.

Alternative Solutions

Today the only workaround is manually toggling autoCompactEnabled on and off mid-session while watching the token count on screen — effectively hand-rolling an ask mode myself. That's tedious and error-prone (easy to forget to re-enable, or misjudge the moment and get compaction anyway).

Priority

High - Significant impact on productivity

Feature Category

Configuration and settings

Use Case Example

On a Bedrock-hosted setup (Claude via Bedrock, autoCompactWindow set to 200000), auto-compact has fired anywhere from ~100k to ~168k tokens of loaded context — no consistent relationship to the configured window (see #86863, #85205, #86789, #85483, #84738 for related reports of early/inconsistent firing). Because of this I now manually flip autoCompactEnabled off, watch the token count accumulate, and switch it back on only when I judge the moment safe — I'm making a better real-time call than the built-in trigger, using nothing but the same on-screen counter the harness has access to. An ask mode would let the harness surface that same decision point instead of me having to notice and act on it manually.

Additional Context

Related: #63232 (prior "prompt" proposal, auto-closed without maintainer reasoning), #47145 (root bug this preempts), #85044 (event-based trigger boundaries — complementary, addresses when, not who decides).

This is issue 1 of a 3-part split (part 2 #90138, part 3 #90139) — see also "[FEATURE] Configurable ask breakpoints for autoCompact" and "[FEATURE] Pluggable summarizer skill for autoCompact", filed separately since they're independently useful.

Note: after this issue is filed, copy its issue number into issue 2's "Depends on" line before submitting that one.