feat: add forge conversation list --all and --since flags to list all and / or recent conversations on the machine

[ahundt]

Adds ability to list conversations across workspaces and filter by date ranges forge conversation list and forge list conversation commands.

Motivation

forge conversation list is scoped to the current workspace by design. There is no way to see conversations from all project directories, which makes it impossible to answer "what was I working on recently?" without cd-ing into every project. This PR adds that capability with minimal, backward-compatible changes.

Usage

forge conversation list                        # current workspace only (unchanged)
forge conversation list --all                  # all workspaces
forge conversation list --since 1w             # current workspace, last week
forge conversation list --all --since 3d       # all workspaces, last 3 days
forge conversation list --all --porcelain      # machine-readable, all workspaces
forge list conversation --all --since 1M       # via list subcommand too

The --all flag queries conversations across all project directories (bypasses workspace_id filter). The --since flag accepts durations like 1h, 30m, 3d, 1w, 1M, 1y and filters by updated_at/created_at.

Implementation:

• Add get_all_workspaces_conversations() to ConversationRepository trait, threaded through repo/services/app/API layers
• Add SinceDuration value type with FromStr parser
• 22 new tests covering both flags and duration parsing
• All CI checks pass: clippy -D warnings, string safety lints, fmt

Design Notes

• Backward compatible — both flags are optional, default behavior unchanged
• Minimal surface area — one new trait method, one new CLI type, two new flags
• Consistent — follows same patterns as existing --porcelain flag
• --since filtering is client-side — the repo layer returns conversations ordered by updated_at DESC with the existing max_conversations limit, then --since filters in the UI layer. This keeps the repo API simple and the filtering logic co-located with display logic.

PR Checklist

• Rebased onto latest upstream/main
• All tests pass
• cargo +nightly fmt --all passes
• cargo +nightly clippy -- -D warnings passes
• String safety lints pass (string_slice, indexing_slicing, disallowed_methods)
• No unrelated changes included
• Single squashed commit

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