feat(cli, config, conversation): Implement conversation compaction

[JeanMertz]

Add jp conversation compact command and --compact / -k flag across query, fork, and compact commands, implementing RFD 064 (non-destructive conversation compaction).

The core idea is that compaction is additive: a Compaction overlay event is appended to the conversation stream rather than modifying or deleting any existing events. The projection layer applies overlays at request-build time (Thread::into_parts()), so the LLM sees a reduced view while the full raw history is preserved on disk.

What users can do now:

# Compact with workspace defaults (strip reasoning + tools)
jp conversation compact

# Strip reasoning, keep last 5 turns
jp conversation compact --reasoning --keep-last 5

# Preview without applying
jp conversation compact --dry-run

# Remove all compaction events (undo)
jp conversation compact --reset

# Compact before querying (inline DSL, summarize all but last 3 turns)
jp query -k s:..-3 -- "Continue the task"

# Fork and compact in one step
jp conversation fork --compact

DSL (--compact=SPEC) supports r (reasoning), t (tools), s (summarize) as policy letters, combined with +, and an optional range after :. Examples: s:..-3, r+t, s:5..-3. Bare --compact applies config rules; --compact=SPEC applies inline rules; both forms compose.

Summarization calls the configured LLM and stores the result in the Compaction event as a SummaryPolicy. At projection time the covered turns are replaced with a synthetic request/response pair containing the summary text. Summary range auto-extension prevents partial overlaps with existing summary compactions.

Configuration is under conversation.compaction.rules — a MergedVec<CompactionRuleConfig> where each rule produces one compaction event. The built-in default (strip reasoning + tools) uses discard_when_merged: true so it is replaced the moment any user rule is present. Alternative compaction configurations can be expressed as individual configuration files loaded with --cfg, consistent with JP's existing config layering.

jp conversation print --compacted shows the projected (assistant) view of the conversation, making it easy to verify the effect of compaction.

The IntoPartialAppConfig trait gains a handles parameter so commands can access resolved conversation targets when building their config overrides.

Ref: #57