fix(chat): make user posts stand out, keep agent-to-agent posts quiet

[selfcontained]

What

From live use: the tint behind your own posts was hard to spot in several themes, and agent → agent traffic was drawing more attention than it deserved.

• Your posts: fill raised from 6% to 10% of primary, so the block reads in every theme.
• Agent → agent posts: the violet fill is gone. They keep the indent and muted body from the side-conversation treatment, so they recede as something you're overhearing rather than competing with messages addressed to you.
• No accent bars. An earlier pass added a left rule to tinted posts; it competed with the connected-agent border the sidebar draws down the pane's left edge, so it's out.
• The agent's own prose stays plain, as before.

Feed tests assert against the POST_TINT constants rather than hardcoded class strings.

Checks

pnpm run check, finalize:web, prettier, and the feed tests (38) pass.

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