[P4][SOC-03] Social posting, replies, cross-posting and messaging

Programme: #1
Epic: #28

## Objective
Enable supported social interactions from the unified workspace while preserving destination-specific account, audience, thread and message semantics.

## Scope
- Typed actions for post, reply, repost/share, reaction, delete where supported.
- Cross-post composition with per-destination account/audience/content/attachment preview.
- Private/direct messaging only through explicit provider-specific capabilities.
- Thread/reply target validation immediately before commit.
- Attachment handling, size/type restrictions and upload progress.
- Provider moderation/visibility constraints and action availability.
- Trusted TX-01 approval where policy requires and provider receipt verification.
- CONN-04 support/access matrix per operation/network.

## Correctness rules
- Generic normalization cannot turn a reply into a private message or change audience semantics.
- Cross-posting is multiple explicit destination actions, not one ambient broadcast permission.
- Account/audience/target changes invalidate stale proposals.

## Acceptance criteria
- [ ] Preview shows exact destination network/account/audience/text/attachments for every write.
- [ ] Reply targets the intended current thread/post or fails on stale/deleted target.
- [ ] Private message capability cannot be reached through a public-reply mapping.
- [ ] Cross-post writes are independently journaled/verified per destination.
- [ ] Unsupported provider actions are disabled/handed off rather than simulated.
- [ ] Delete/visibility changes propagate back into the unified feed when provider state confirms them.

## Dependencies
- SOC-01
- TX-01
- CONN-04

**First phase:** P4
**Maturity target:** P7
**Owner:** connectors-domains

No files, tests, or entry points are named. Start by reading SOC-01, TX-01, and CONN-04, then map the provider-specific capabilities and approval or verification boundaries; done means meeting every listed acceptance criterion for previews, stale targets, independent journaling, unsupported actions, and confirmed state changes.

Written by the indexing model from the issue text.