feat(/compress): manual session compression with focus topic — closes #469 (PR #619 by @franksong2702)

POST /api/session/compress with optional focus_topic. Transcript-inline cards: command, running, complete (collapsible green), reference. /compact alias kept. Fixes: var(--green) undefined color, focus_topic 500-char cap. Independent review by @nesquena (4 passes).

README.md

@@ -436,7 +436,7 @@ across 53 test files.
 
 ### Slash commands
 - Type `/` in the composer for autocomplete dropdown
-- Built-in: `/help`, `/clear`, `/model <name>`, `/workspace <name>`, `/new`, `/usage`, `/theme`, `/compact`
+- Built-in: `/help`, `/clear`, `/compress [focus topic]`, `/compact` (alias), `/model <name>`, `/workspace <name>`, `/new`, `/usage`, `/theme`
 - Arrow keys navigate, Tab/Enter select, Escape closes
 - Unrecognized commands pass through to the agent