fix: preserve chatMessage subject on reads and add message send --subject

[amanjoshicodes]

Problem

ChatMessage in src/models/message.rs has no subject field, so the subject of a channel root message — which Graph returns on both GET .../messages/{id} and the list endpoint — is silently dropped during deserialization. A message posted with a subject reads back without one:

$ teams message send --team TEAM_ID --channel CHANNEL_ID --subject "Release plan" --body "Details inside."   # (not possible before this PR)
$ teams message get --team TEAM_ID --channel CHANNEL_ID MESSAGE_ID --output json | jq .data.subject
null        # v0.6.0 — Graph returns "Release plan", the CLI loses it

There was also no way to set a subject on send.

Change

• ChatMessage gains subject: Option<String> (skip-serializing when absent, so messages without a subject gain no "subject": null noise). message get and message list now return what Graph stores.
• SendMessageRequest gains the top-level subject, exposed as teams message send --subject TEXT. Channel sends only: chat messages have no subject, so clap rejects --subject with --chat or without --channel (exit 2) before anything is sent.
• Docs: README, docs/command-reference.md, docs/man/teams.1, CHANGELOG entry under Unreleased. No version bump (per the release-automation convention).

Tests

• Model round-trips: subject survives deserialize/serialize; absent subject stays omitted; SendMessageRequest serializes subject at the top level.
• CLI regressions: message send --help advertises --subject <SUBJECT>; --subject with --chat and --subject without --channel both fail with exit 2.
• cargo fmt -- --check, cargo clippy --all-targets --all-features -- -D warnings, cargo test --all-targets (313 tests) all green.

Live verification (real tenant, delegated auth)

Send with the patched binary, then read back three ways:

$ teams message send --team TEAM_ID --channel CHANNEL_ID --subject "CLI subject test" --body "[TESTING] ..." --output json | jq '{id: .data.id, subject: .data.subject}'
{ "id": "MESSAGE_ID", "subject": "CLI subject test" }

$ teams message get --team TEAM_ID --channel CHANNEL_ID MESSAGE_ID --output json | jq .data.subject
"CLI subject test"                  # patched binary; raw Graph GET agrees

$ teams-v0.6.0 message get --team TEAM_ID --channel CHANNEL_ID MESSAGE_ID --output json | jq .data.subject
null                                # released binary on the same message

message list on the same channel shows the subject on the patched binary as well.